diff --git a/cmd/mackerel-plugin-jsonl/main.go b/cmd/mackerel-plugin-jsonl/main.go
index c21aab7..1d3b166 100644
--- a/cmd/mackerel-plugin-jsonl/main.go
+++ b/cmd/mackerel-plugin-jsonl/main.go
@@ -28,6 +28,7 @@ type Opt struct {
 	filterByte          *[]byte
 	ignoreByte          *[]byte
 	paths               [][]string
+	flatPaths           bool
 	duration            float64
 }
 
diff --git a/cmd/mackerel-plugin-jsonl/main_test.go b/cmd/mackerel-plugin-jsonl/main_test.go
index f1c6008..e6d6a4c 100644
--- a/cmd/mackerel-plugin-jsonl/main_test.go
+++ b/cmd/mackerel-plugin-jsonl/main_test.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/monitoring-forge/followparser"
+	"github.com/stretchr/testify/require"
 )
 
 func generateJSONLFile(b testing.TB, dir, filename string, numLines int) error {
@@ -90,7 +91,7 @@ func initParserForTest(b testing.TB, tmpDir string) (*followparser.Parser, *Opt)
 	return fp, opt
 }
 
-func internalBenchmarkParse(b *testing.B, numLines int, doOutput bool) {
+func internalBenchmarkParse(b *testing.B, numLines int, doOutput, useEachKey bool) {
 	tmpDir := b.TempDir()
 	prefix := "json"
 	logFileName := "json.log"
@@ -107,44 +108,39 @@ func internalBenchmarkParse(b *testing.B, numLines int, doOutput bool) {
 	for b.Loop() {
 		b.StopTimer()
 		err := resetFollowParserStateFile(b, tmpDir, logFileName, prefix)
-		if err != nil {
-			b.Fatalf("resetFollowParserStateFile failed: %v", err)
-		}
+		require.NoError(b, err, "resetFollowParserStateFile failed")
 		b.StartTimer()
 		fp, opt := initParserForTest(b, tmpDir)
+		if useEachKey {
+			opt.flatPaths = false
+		}
 		parsed, err := fp.Parse(
 			posFile,
 			logFile,
 		)
-		if err != nil {
-			b.Fatalf("Parse failed: %v", err)
-		}
+		require.NoError(b, err, "Parse failed")
 		if doOutput {
 			output := opt.output()
-			if output == "" {
-				b.Fatalf("output is empty")
-			}
+			require.NotEmpty(b, output, "output is empty")
 		}
 		b.StopTimer()
-		if parsed == nil {
-			b.Fatalf("Parse returned nil parsed data")
-		}
-		if len(parsed) != 1 {
-			b.Fatalf("Parse returned unexpected number of parsed data: got %d, want 1", len(parsed))
-		}
-		if parsed[0].Rows != numLines {
-			b.Fatalf("Parse returned unexpected number of rows: got %d, want %d", parsed[0].Rows, numLines)
-		}
+		require.NotNil(b, parsed, "parsed data is nil")
+		require.Equal(b, 1, len(parsed), "unexpected number of parsed data")
+		require.Equal(b, numLines, parsed[0].Rows, "unexpected number of rows in parsed data")
 		b.StartTimer()
 	}
 }
 
 // generate 100k JSONL file and parse benchmark
 func BenchmarkMainParse_jsonl(b *testing.B) {
-	internalBenchmarkParse(b, 100_000, false)
+	internalBenchmarkParse(b, 100_000, false, false)
+}
+
+func BenchmarkMainParse_jsonl_eachkey(b *testing.B) {
+	internalBenchmarkParse(b, 100_000, false, true)
 }
 
 // generate 100k JSONL file and parse benchmark
 func BenchmarkMainParse_parse_and_output(b *testing.B) {
-	internalBenchmarkParse(b, 100_000, true)
+	internalBenchmarkParse(b, 100_000, true, false)
 }
diff --git a/cmd/mackerel-plugin-jsonl/parser.go b/cmd/mackerel-plugin-jsonl/parser.go
index 4f02bb5..a4223aa 100644
--- a/cmd/mackerel-plugin-jsonl/parser.go
+++ b/cmd/mackerel-plugin-jsonl/parser.go
@@ -2,11 +2,14 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"log"
 
 	"github.com/buger/jsonparser"
 )
 
+var errFlatPathsFound = errors.New("all flat JSON paths found")
+
 func (opt *Opt) jsonParsed(idx int, value []byte, vt jsonparser.ValueType, err error) {
 	if err != nil {
 		log.Printf("error: %v", err)
@@ -37,10 +40,45 @@ func (opt *Opt) Parse(b []byte) error {
 		}
 	}
 
-	jsonparser.EachKey(b, opt.jsonParsed, opt.paths...)
+	if opt.flatPaths {
+		opt.parseFlat(b)
+	} else {
+		jsonparser.EachKey(b, opt.jsonParsed, opt.paths...)
+	}
 	return nil
 }
 
+// parseFlat reads only root-level keys. ObjectEach decodes escaped key names,
+// so comparisons here use the same names as paths passed to EachKey.
+func (opt *Opt) parseFlat(b []byte) {
+	var found uint64
+	remaining := len(opt.paths)
+	err := jsonparser.ObjectEach(b, func(key, value []byte, valueType jsonparser.ValueType, _ int) error {
+		for i, path := range opt.paths {
+			bit := uint64(1) << i
+			if found&bit != 0 || !bytes.Equal(key, []byte(path[0])) {
+				continue
+			}
+			found |= bit
+			remaining--
+			opt.jsonParsed(i, value, valueType, nil)
+		}
+		if remaining == 0 {
+			return errFlatPathsFound
+		}
+		return nil
+	})
+	if err != nil && err != errFlatPathsFound { //nolint:errorlint
+		// Keep EachKey's behavior for non-object and malformed input. Skip values
+		// already delivered by ObjectEach so aggregators never count them twice.
+		jsonparser.EachKey(b, func(i int, value []byte, valueType jsonparser.ValueType, err error) {
+			if i < 0 || found&(uint64(1)<<i) == 0 {
+				opt.jsonParsed(i, value, valueType, err)
+			}
+		}, opt.paths...)
+	}
+}
+
 func (opt *Opt) Finish(duration float64) {
 	opt.duration = duration
 }
diff --git a/cmd/mackerel-plugin-jsonl/parser_test.go b/cmd/mackerel-plugin-jsonl/parser_test.go
index 5735a6d..4c306bb 100644
--- a/cmd/mackerel-plugin-jsonl/parser_test.go
+++ b/cmd/mackerel-plugin-jsonl/parser_test.go
@@ -1,11 +1,115 @@
 package main
 
 import (
+	"reflect"
 	"testing"
 
 	"github.com/monitoring-forge/sampdo"
+	"github.com/stretchr/testify/require"
 )
 
+func TestParseFlatMatchesEachKey(t *testing.T) {
+	tests := []struct {
+		name  string
+		paths [][]string
+		line  string
+	}{
+		{
+			name:  "root keys after nested values",
+			paths: [][]string{{"time"}, {"status"}, {"reqtime"}},
+			line:  `{"extra":{"status":"nested"},"time":"now","status":"200","reqtime":0.5}`,
+		},
+		{
+			name:  "escaped key and value",
+			paths: [][]string{{"status"}, {"a.b"}},
+			line:  `{"sta\u0074us":"ok\nnext","a.b":"dotted"}`,
+		},
+		{
+			name:  "duplicate keys use first value",
+			paths: [][]string{{"status"}, {"time"}},
+			line:  `{"status":"200","status":"500","time":"now"}`,
+		},
+		{
+			name:  "duplicate requested paths",
+			paths: [][]string{{"status"}, {"status"}},
+			line:  `{"status":"200","status":"500"}`,
+		},
+		{
+			name:  "missing and null values",
+			paths: [][]string{{"status"}, {"time"}, {"missing"}},
+			line:  `{"status":null,"time":"now"}`,
+		},
+		{
+			name:  "compound values",
+			paths: [][]string{{"status"}, {"time"}, {"reqtime"}},
+			line:  `{"status":{"code":200},"time":[1,2],"reqtime":true}`,
+		},
+		{
+			name:  "root array uses generic parser",
+			paths: [][]string{{"status"}, {"time"}},
+			line:  `[{"status":"200"},{"time":"now"}]`,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			newOpt := func() *Opt {
+				opt := &Opt{}
+				for _, path := range tc.paths {
+					opt.aggregatorFunctions = append(opt.aggregatorFunctions, &AggregatorFunction{
+						jsonKey: path, aggregator: "group_by", groupBy: map[string]int{},
+					})
+				}
+				opt.setupPaths()
+				return opt
+			}
+
+			fast := newOpt()
+			require.True(t, fast.flatPaths, "expected flat path optimization")
+			reference := newOpt()
+			reference.flatPaths = false
+			for _, opt := range []*Opt{fast, reference} {
+				err := opt.Parse([]byte(tc.line))
+				require.NoError(t, err, "Parse failed")
+			}
+			for i := range tc.paths {
+				if !reflect.DeepEqual(fast.aggregatorFunctions[i].groupBy, reference.aggregatorFunctions[i].groupBy) {
+					t.Errorf("path %v: fast=%v, EachKey=%v", tc.paths[i], fast.aggregatorFunctions[i].groupBy, reference.aggregatorFunctions[i].groupBy)
+				}
+			}
+		})
+	}
+}
+
+func TestSetupPathsSelectsFlatOnlyForSmallRootKeySets(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		paths [][]string
+		want  bool
+	}{
+		{"one root key", [][]string{{"status"}}, true},
+		{"nested key", [][]string{{"status", "code"}}, false},
+		{"array key", [][]string{{"items", "[0]"}}, false},
+		{"no keys", nil, false},
+		{"64 keys", make([][]string, 64), true},
+		{"65 keys", make([][]string, 65), false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			opt := &Opt{}
+			for _, path := range tc.paths {
+				if path == nil {
+					path = []string{"status"}
+				}
+				opt.aggregatorFunctions = append(opt.aggregatorFunctions, &AggregatorFunction{jsonKey: path})
+			}
+			opt.setupPaths()
+			if opt.flatPaths != tc.want {
+				t.Errorf("flatPaths=%v, want %v", opt.flatPaths, tc.want)
+			}
+		})
+	}
+}
+
 func TestParser_Parse(t *testing.T) {
 	opt := &Opt{
 		aggregatorFunctions: []*AggregatorFunction{
diff --git a/cmd/mackerel-plugin-jsonl/reader.go b/cmd/mackerel-plugin-jsonl/reader.go
index c44ef56..ddd7471 100644
--- a/cmd/mackerel-plugin-jsonl/reader.go
+++ b/cmd/mackerel-plugin-jsonl/reader.go
@@ -48,7 +48,10 @@ func (af *AggregatorFunction) appendData(b []byte) error {
 		if err != nil {
 			return err
 		}
-		af.percentiles.Append(floatValue)
+		err = af.percentiles.Append(floatValue)
+		if err != nil {
+			return err
+		}
 	}
 
 	return nil
@@ -133,8 +136,13 @@ func (p *Opt) setupFilterBytes() {
 
 func (p *Opt) setupPaths() {
 	paths := make([][]string, 0, len(p.aggregatorFunctions))
+	// parseFlat uses a uint64 to track paths already found in each line.
+	p.flatPaths = len(p.aggregatorFunctions) > 0 && len(p.aggregatorFunctions) <= 64
 	for _, af := range p.aggregatorFunctions {
 		paths = append(paths, af.jsonKey)
+		if len(af.jsonKey) != 1 {
+			p.flatPaths = false
+		}
 	}
 	p.paths = paths
 }