perf: async lowering is 85% of asyncpipe — Perry pays 6.8x what Node pays for the same async/sync difference #8008

Summary

Async lowering is 85% of asyncpipe, and Perry pays 6.8× what Node pays for the same async/sync difference.

Node pays ~7% for that same difference (0.096 → 0.090 s CPU). Perry pays 6.8×.

This issue exists because the components were filed and closed individually while the headline never had a ticket — so nothing tracks whether the gap as a whole is closing.

Decomposition (measured, gc-handoff/ASYNC2-NOTES.md)

Reading it: async topology (24,000 activations × 3 async fns × 2 awaits) ≈ 366 M; carrying an object rather than a number ≈ +519 M; Promise.all fan-out ≈ 570 M.

Leaf profile of the 1× program (14 aggregated xctrace runs, 2034 samples):

0.9% of this program is the user's code.

⚠️ The headline predates three merged fixes — re-measure before acting

Since that decomposition was taken, #7927 (thenable probe), #7924 (Promise.all fast arm) and #7939 (async activation boxes released) all landed, and asyncpipe went 1.63× → 1.05× node on the quiet mini. Two of the four decomposition buckets have been attacked directly.

So the first task is to re-run the probe set on current main and republish the table. The 85% figure is the reason this issue exists; it is not necessarily still 85%. Do not open a fix against the old numbers.

What is known to be not the answer

• Lifting main's aggregation loop out of the async function changes nothing (−0.4%). CLAUDE.md's "the transform boxes every body local as Any" weakness is real but is not where this program's time is — main loops 24,000 times against 72,000 promise activations.
• Removing Map/Set from the hot path: −4.0%. Not it either.
• ★ a2_nostr1x (template literal replaced by a constant) is a discarded probe — it came out +85% slower because replacing the literal flips the program into a different GC regime. Do not reuse it.
• ★ asyncpipe_big.ts is not a valid scaled proxy (bench: asyncpipe@120 sits exactly on a nursery trigger boundary and is not a usable A/B target #7926): 120 batches → 0 GC cycles, 240 → 2 copying minors, 1200 → GC-dominated. Any scaling must be shown to preserve the mechanism under study.

Acceptance

• A republished decomposition table on current main.
• If a gap remains, a named mechanism with a sized lever — not "async is slow".
• Report instructions retired (load-independent) alongside any wall clock; the dev box has run at load 30–200 and cannot resolve 5%.

Related

#7909, #7910, #7911, #7933 (all closed — the components), #7926 (the benchmark's trigger cliff).