diff --git a/CHANGELOG.md b/CHANGELOG.md
index dabfb17a1..4df0a62b9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,7 @@
 * [#3071](https://github.com/ruby-grape/grape/pull/3071): Skip the query parse when building the params of a request with no query string - [@ericproulx](https://github.com/ericproulx).
 * [#3072](https://github.com/ruby-grape/grape/pull/3072): Answer the `Accept` lists axios, jQuery and HTTPie send without negotiating them on every request - [@ericproulx](https://github.com/ericproulx).
 * [#3073](https://github.com/ruby-grape/grape/pull/3073): Let a `rescue_from :all` in a namespace or mounted API outrank the enclosing scope's handlers - [@ericproulx](https://github.com/ericproulx).
+* [#3074](https://github.com/ruby-grape/grape/pull/3074): Speed up `declared` by working out each param's key and empty value when the endpoint compiles - [@ericproulx](https://github.com/ericproulx).
 * [#3075](https://github.com/ruby-grape/grape/pull/3075): Validate the params of an optional Hash scope without the attributes iterator - [@ericproulx](https://github.com/ericproulx).
 * [#3076](https://github.com/ruby-grape/grape/pull/3076): Skip the query merge when building the params of a body sent without a query string - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
diff --git a/lib/grape/declared_params.rb b/lib/grape/declared_params.rb
new file mode 100644
index 000000000..5a9a4c89c
--- /dev/null
+++ b/lib/grape/declared_params.rb
@@ -0,0 +1,114 @@
+# frozen_string_literal: true
+
+module Grape
+  # The params an endpoint declares, as +declared+ filters a Hash by them.
+  #
+  # What +declared+ answers for a param depends on the request only for its
+  # value. The key it is filed under -- its name, or the one +as:+ renamed it
+  # to -- and the empty value it stands for when it was not passed -- an
+  # Array, a Set or a Hash, after its documented type -- are settled by the
+  # declaration. Both were worked out again on every call, for every param:
+  # the path to the param built to look its new name up, its documented name
+  # spelled out to look its type up, that type matched against the ones that
+  # have an empty value. They are worked out here once, when the endpoint
+  # compiles, as Validations::UndeclaredParams is.
+  class DeclaredParams
+    extend Grape::Util::FreezeOnNew
+
+    # A declared param: the +name+ it is read by, the +key+ and +string_key+
+    # it is filed under, the params declared under it (+children+, nil for a
+    # param declared without a block), the +empty_value+ it stands for when
+    # it was not passed (nil when its type has none), and the +scope+ that
+    # declared it, whose +given+ +evaluate_given+ asks about.
+    Param = Data.define(:name, :key, :string_key, :children, :empty_value, :scope)
+
+    EMPTY_HASH = {}.freeze
+    EMPTY_ARRAY = [].freeze
+    EMPTY_SET = Set.new.freeze
+    private_constant :EMPTY_HASH, :EMPTY_ARRAY, :EMPTY_SET
+
+    # +all+ are the params of the endpoint and of its namespaces, +own+ the
+    # ones of the endpoint's own +params+ block, which is what
+    # +include_parent_namespaces: false+ keeps.
+    attr_reader :all, :own, :contract_key_maps
+
+    # @param declared_params [Array<Validations::ParamsScope::Attr>] the
+    #   endpoint's declared params, its namespaces' included
+    # @param own_declared_params [Array<Validations::ParamsScope::Attr>] the
+    #   ones its own +params+ block declares
+    # @param route_params [Hash] the endpoint's documented params, by full name
+    # @param renamed_params [Hash] the names +as:+ gave, by the path of the
+    #   param they rename
+    # @param contract_key_maps [Array<Dry::Schema::KeyMap>] the keys of the
+    #   endpoint's contracts
+    def initialize(declared_params:, own_declared_params:, route_params:, renamed_params:, contract_key_maps:)
+      @route_params = route_params
+      @renamed_params = renamed_params
+      @contract_key_maps = contract_key_maps
+      # An endpoint's own params are among all of them, and compile once.
+      compiled = {}.compare_by_identity
+      @all = declared_params.flat_map { |attr| compiled[attr] ||= compile(attr, nil, nil) }.freeze
+      @own = own_declared_params.flat_map { |attr| compiled[attr] ||= compile(attr, nil, nil) }.freeze
+    end
+
+    private
+
+    # The params of a ParamsScope::Attr, whose key is a name, or a Hash of
+    # names to the Attrs declared under them. +parent_name+ is the enclosing
+    # param as the route documents it (+"a[b]"+) and +parent_path+ the names
+    # leading to it (+%w[a b]+), both nil at the root.
+    def compile(attr, parent_name, parent_path)
+      declared = attr.key
+      return [param(declared, nil, attr.scope, parent_name, parent_path)] unless declared.is_a?(Hash)
+
+      declared.map { |name, children| param(name, children, attr.scope, parent_name, parent_path) }
+    end
+
+    def param(name, children, scope, parent_name, parent_path)
+      path = [*parent_path, name.to_s]
+      full_name = parent_name ? "#{parent_name}[#{name}]" : name.to_s
+      key = @renamed_params[path] || name
+      Param.new(
+        name:,
+        # A name read from +configuration+ is nil until the API is mounted
+        # with one, and an API compiles unmounted too.
+        key: key.respond_to?(:to_sym) ? key.to_sym : key,
+        string_key: -key.to_s,
+        children: children&.flat_map { |child| compile(child, full_name, path) }.freeze,
+        empty_value: empty_value_for(full_name),
+        scope:
+      )
+    end
+
+    # A Hash is only empty when no param is declared under it: one that
+    # declares some stands for a Hash of them instead, each nil or empty in
+    # turn.
+    def empty_value_for(full_name)
+      type = documented_type(full_name)
+      return unless type
+      return EMPTY_HASH if type == 'Hash' && no_child_params?("#{full_name}[")
+      return EMPTY_ARRAY if array_type?(type)
+
+      EMPTY_SET if type.start_with?('#<Set', 'Set')
+    end
+
+    # Nil for a type that is not spelled as a String, which has no empty
+    # value: a +desc+ may document a param of its own with a Class, or with
+    # no Hash of details at all.
+    def documented_type(full_name)
+      details = @route_params[full_name]
+      type = details[:type] if details.is_a?(Hash)
+      type if type.is_a?(String)
+    end
+
+    # +Array+ and +Array[String]+, but not the Array of types a param
+    # declared with +types:+ documents.
+    def array_type?(type)
+      type == 'Array' || (type.start_with?('[') && !type.include?(','))
+    end
+
+    def no_child_params?(prefix)
+      @route_params.keys.none? { |documented| documented.to_s.start_with?(prefix) }
+    end
+  end
+end
diff --git a/lib/grape/declared_params_handler.rb b/lib/grape/declared_params_handler.rb
index e8142d7bb..12c2ef4cf 100644
--- a/lib/grape/declared_params_handler.rb
+++ b/lib/grape/declared_params_handler.rb
@@ -9,17 +9,10 @@ def initialize(include_missing: true, evaluate_given: false, stringify: false, c
       @contract_key_map = contract_key_map
     end
 
-    def call(passed_params, declared_params, route_params, renamed_params)
-      recursive_declared(
-        passed_params,
-        declared_params:,
-        route_params:,
-        renamed_params:
-      )
-    end
-
-    private
-
+    # @param passed_params [Hash, Array] the params to filter
+    # @param declared_params [Array<Grape::DeclaredParams::Param>] the params
+    #   to keep of them
+    #
     # An element of an Array scope that is not a Hash has no keys to filter,
     # so it is returned as it came in, the way a leaf value is. Validation
     # lets one through wherever nothing in the scope is required of it: an
@@ -27,15 +20,13 @@ def call(passed_params, declared_params, route_params, renamed_params)
     # all optional checks nothing on an element that is not a Hash. Filtering
     # it anyway meant building an empty copy of it, which raised for +nil+,
     # +false+ and numbers and indexed a String with a Symbol.
-    def recursive_declared(passed_params, declared_params:, route_params:, renamed_params:, params_nested_path: [])
+    def call(passed_params, declared_params)
       return passed_params unless passed_params.is_a?(Array) || passed_params.respond_to?(:key?)
 
       res = if passed_params.is_a?(Array)
-              passed_params.map do |passed_param|
-                recursive_declared(passed_param, declared_params:, params_nested_path:, renamed_params:, route_params:)
-              end
+              passed_params.map { |passed_param| call(passed_param, declared_params) }
             else
-              declared_hash(passed_params, declared_params:, params_nested_path:, renamed_params:, route_params:)
+              declared_hash(passed_params, declared_params)
             end
 
       @contract_key_map.each { |key_map| key_map.write(passed_params, res) }
@@ -43,102 +34,38 @@ def recursive_declared(passed_params, declared_params:, route_params:, renamed_p
       res
     end
 
-    def declared_hash(passed_params, declared_params:, params_nested_path:, renamed_params:, route_params:)
-      declared_params.each_with_object(passed_params.class.new) do |declared_param_attr, memo|
-        next if @evaluate_given && !declared_param_attr.scope.attr_meets_dependency?(passed_params)
-
-        declared_hash_attr(
-          passed_params,
-          declared_param: declared_param_attr.key,
-          params_nested_path:,
-          memo:,
-          renamed_params:,
-          route_params:
-        )
-      end
-    end
-
-    def declared_hash_attr(passed_params, declared_param:, params_nested_path:, memo:, renamed_params:, route_params:)
-      return declare_leaf(passed_params, declared_param:, params_nested_path:, memo:, renamed_params:, route_params:) unless declared_param.is_a?(Hash)
-
-      declared_param.each_pair do |parent, children|
-        declare_nested(passed_params, parent:, children:, params_nested_path:, memo:, renamed_params:, route_params:)
-      end
-    end
-
-    def declare_nested(passed_params, parent:, children:, params_nested_path:, memo:, renamed_params:, route_params:)
-      return unless @include_missing || passed_params.key?(parent)
-
-      memo_key = build_memo_key(params_nested_path, parent, renamed_params)
-      passed_children = passed_params[parent] || passed_params.class.new
-      nested_path = nested_path_for(params_nested_path, parent)
-      # A value that is neither a Hash nor an Array where children are declared
-      # -- left unvalidated by a +given+ whose dependency is not met, say -- is
-      # returned as it came in, as #recursive_declared returns an element like
-      # it.
-      has_passed_children = !passed_children.respond_to?(:any?) || passed_children.any?
-
-      memo[memo_key] = handle_passed_param(nested_path, route_params:, has_passed_children:) do
-        recursive_declared(
-          passed_children,
-          declared_params: children,
-          params_nested_path: nested_path,
-          renamed_params:,
-          route_params:
-        )
-      end
-    end
+    private
 
-    # The declared param has no children. Find its value or set it to nil.
-    def declare_leaf(passed_params, declared_param:, params_nested_path:, memo:, renamed_params:, route_params:)
-      return unless @include_missing || (passed_params.respond_to?(:key?) && passed_params.key?(declared_param))
+    def declared_hash(passed_params, declared_params)
+      declared_params.each_with_object(passed_params.class.new) do |declared_param, memo|
+        next if @evaluate_given && !declared_param.scope.attr_meets_dependency?(passed_params)
 
-      memo_key = build_memo_key(params_nested_path, declared_param, renamed_params)
-      passed_param = passed_params[declared_param]
+        name = declared_param.name
+        next unless @include_missing || passed_params.key?(name)
 
-      memo[memo_key] = passed_param || handle_passed_param(nested_path_for(params_nested_path, declared_param), route_params:) do
-        passed_param
+        memo[@stringify ? declared_param.string_key : declared_param.key] = declared_value(passed_params[name], declared_param, passed_params)
       end
     end
 
-    # The lookup key is the param's whole path, built fresh -- two Arrays and a
-    # String per declared param -- and only an API using +as:+ has anything to
-    # find with it, so the common empty table is not asked.
-    def build_memo_key(params_nested_path, declared_param, renamed_params)
-      renamed_param_name = renamed_params[nested_path_for(params_nested_path, declared_param)] unless renamed_params.empty?
-      param = renamed_param_name || declared_param
-      @stringify ? param.to_s : param.to_sym
-    end
-
-    def nested_path_for(parent_path, key)
-      parent_path + [key.to_s]
-    end
-
-    def handle_passed_param(params_nested_path, route_params:, has_passed_children: false)
-      return yield if has_passed_children
-
-      key = params_nested_path.first
-      key += "[#{params_nested_path[1..].join('][')}]" if params_nested_path.size > 1
+    # What a declared param is filed as: its value, or the empty value its
+    # type has when that is nil, a fresh one every time since the caller may
+    # fill it.
+    def declared_value(passed_param, declared_param, passed_params)
+      children = declared_param.children
+      return declared_children(passed_param || passed_params.class.new, declared_param, children) if children
 
-      type = route_params.dig(key, :type)
-      return yield if type.nil?
-
-      empty_value_for(type, key, route_params) || yield
+      passed_param || declared_param.empty_value&.dup || passed_param
     end
 
-    # The empty value a param of +type+ declares when it was not passed, or
-    # nil when +type+ has none. A fresh one every time: the caller may fill it.
-    def empty_value_for(type, key, route_params)
-      return {} if type == 'Hash' && no_child_params?(route_params, "#{key}[")
-      return [] if type == 'Array' || (type.start_with?('[') && !type.include?(','))
-
-      Set.new if type.start_with?('#<Set', 'Set')
-    end
-
-    # The prefix is built once by the caller: interpolated inside the block, it
-    # was a new String for every param on the route.
-    def no_child_params?(route_params, prefix)
-      route_params.keys.none? { |k| k.start_with?(prefix) }
+    # A param with children is filed as what #call keeps of its value, a Hash
+    # of the same class as the params around it when it was not passed.
+    #
+    # A value that is neither a Hash nor an Array where children are declared
+    # -- left unvalidated by a +given+ whose dependency is not met, say -- is
+    # returned as it came in, as #call returns an element like it.
+    def declared_children(passed_children, declared_param, children)
+      has_passed_children = !passed_children.respond_to?(:any?) || passed_children.any?
+      (declared_param.empty_value&.dup unless has_passed_children) || call(passed_children, children)
     end
   end
 end
diff --git a/lib/grape/dsl/declared.rb b/lib/grape/dsl/declared.rb
index c0142aacb..5668b1654 100644
--- a/lib/grape/dsl/declared.rb
+++ b/lib/grape/dsl/declared.rb
@@ -23,13 +23,9 @@ def initialize(msg = '#declared is not available prior to parameter validation')
       def declared(passed_params, include_parent_namespaces: true, include_missing: true, evaluate_given: false, stringify: false)
         raise MethodNotYetAvailable unless before_filter_passed
 
-        contract_key_map = inheritable_setting.contract_key_maps
-        handler = DeclaredParamsHandler.new(include_missing:, evaluate_given:, stringify:, contract_key_map:)
-        declared_params = include_parent_namespaces ? inheritable_setting.route_declared_params : (inheritable_setting.declared_params.last || [])
-        renamed_params = inheritable_setting.route_renamed_params
-        route_params = config.params
-
-        handler.call(passed_params, declared_params, route_params, renamed_params)
+        declared_params = compiled_declared_params
+        handler = DeclaredParamsHandler.new(include_missing:, evaluate_given:, stringify:, contract_key_map: declared_params.contract_key_maps)
+        handler.call(passed_params, include_parent_namespaces ? declared_params.all : declared_params.own)
       end
     end
   end
diff --git a/lib/grape/endpoint.rb b/lib/grape/endpoint.rb
index ad0d304fb..26e0e7829 100644
--- a/lib/grape/endpoint.rb
+++ b/lib/grape/endpoint.rb
@@ -265,6 +265,12 @@ def become_prototype(state)
 
     attr_reader :before_filter_passed
 
+    # What #declared filters params by. Named so that no helper is likely to
+    # take its place: a helper overrides any method of the endpoint.
+    def compiled_declared_params
+      @state.declared_params
+    end
+
     # The answer of an allowed-methods route, which the router hands a request
     # for a method its path has no route for: the Allow header, with a 204 to an
     # auto-OPTIONS and a 405 to anything else.
@@ -352,7 +358,8 @@ def compile!
         app:,
         callbacks: callbacks.each_value.all?(&:empty?) ? nil : callbacks.freeze,
         build_params_with: inheritable_setting.build_params_with,
-        undeclared_params: build_undeclared_params
+        undeclared_params: build_undeclared_params,
+        declared_params: build_declared_params
       )
       @prototype = build_prototype(build_helpers)
     end
@@ -367,6 +374,17 @@ def build_undeclared_params
       )
     end
 
+    def build_declared_params
+      setting = inheritable_setting
+      Grape::DeclaredParams.new(
+        declared_params: setting.route_declared_params,
+        own_declared_params: setting.declared_params.last || [],
+        route_params: config.params,
+        renamed_params: setting.route_renamed_params,
+        contract_key_maps: setting.contract_key_maps
+      )
+    end
+
     # The object each request is copied from: this endpoint's State, and none
     # of the route-building state (+@prototype+, +@endpoints+, +@namespace+,
     # +@routes+) the request path never reads.
diff --git a/lib/grape/endpoint/state.rb b/lib/grape/endpoint/state.rb
index af6900449..49b6188dc 100644
--- a/lib/grape/endpoint/state.rb
+++ b/lib/grape/endpoint/state.rb
@@ -4,12 +4,13 @@ module Grape
   class Endpoint
     # Everything an endpoint's requests read from it and none of them changes.
     # Each request's copy of the endpoint holds this and its own state, and
-    # nothing else of the endpoint. +app+, +callbacks+, +build_params_with+ and
-    # +undeclared_params+ are filled in by Endpoint#compile!.
+    # nothing else of the endpoint. +app+, +callbacks+, +build_params_with+,
+    # +undeclared_params+ and +declared_params+ are filled in by
+    # Endpoint#compile!.
     State = Data.define(:inheritable_setting, :options, :config, :source, :options_route_enabled, :app, :callbacks, :build_params_with,
-                        :undeclared_params) do
+                        :undeclared_params, :declared_params) do
       def initialize(inheritable_setting:, options:, config:, source:, options_route_enabled: false, app: nil, callbacks: nil,
-                     build_params_with: nil, undeclared_params: nil)
+                     build_params_with: nil, undeclared_params: nil, declared_params: nil)
         super
       end
     end
diff --git a/spec/grape/endpoint/declared_spec.rb b/spec/grape/endpoint/declared_spec.rb
index 94abbf185..c2cc307d0 100644
--- a/spec/grape/endpoint/declared_spec.rb
+++ b/spec/grape/endpoint/declared_spec.rb
@@ -953,4 +953,84 @@
       expect(JSON.parse(last_response.body)).to eq('shipping' => nil, 'address' => 'somewhere')
     end
   end
+
+  describe 'a param a desc documents in its own way' do
+    let(:app) do
+      Class.new(Grape::API) do
+        format :json
+
+        desc 'documented by hand', params: { 'limit' => { type: Integer }, 'note' => 'free text', 'ids' => { type: 'Array' } }
+        params do
+          optional :limit, type: Integer
+          optional :note, type: String
+          optional :ids, type: String
+        end
+        get('/') { declared(params) }
+      end
+    end
+
+    # The type decides the empty value a param that was not passed stands
+    # for, and only a type spelled as a String names one.
+    it 'has the empty value of the type the desc names, and none when it names no type as a String' do
+      get '/'
+      expect(last_response.status).to eq(200)
+      expect(JSON.parse(last_response.body)).to eq('limit' => nil, 'note' => nil, 'ids' => [])
+    end
+  end
+
+  describe 'a param named by the configuration' do
+    let(:api) do
+      Class.new(Grape::API) do
+        format :json
+
+        params do
+          requires configuration[:name], type: String
+          optional :tags, type: Array
+        end
+        get('/') { declared(params) }
+      end
+    end
+    let(:app) do
+      mounted = api
+      Class.new(Grape::API) { mount mounted, with: { name: :title } }
+    end
+
+    it 'compiles before the API is mounted, when it has no name yet' do
+      expect { api.compile! }.not_to raise_error
+    end
+
+    it 'is filed under the name the API is mounted with' do
+      get '/', title: 'grape', junk: 1
+      expect(last_response.status).to eq(200)
+      expect(JSON.parse(last_response.body)).to eq('title' => 'grape', 'tags' => [])
+    end
+  end
+
+  describe 'an empty value' do
+    let(:app) do
+      Class.new(Grape::API) do
+        format :json
+
+        params do
+          optional :tags, type: Array
+          optional :meta, type: Hash
+          optional :ids, type: Set
+        end
+        get '/' do
+          first = declared(params)
+          first[:tags] << 'filled'
+          first[:meta][:filled] = true
+          first[:ids] << 1
+          declared(params).transform_values(&:size)
+        end
+      end
+    end
+
+    it 'is a new one for every call' do
+      2.times do
+        get '/'
+        expect(JSON.parse(last_response.body)).to eq('tags' => 0, 'meta' => 0, 'ids' => 0)
+      end
+    end
+  end
 end