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performance-investigation

★ 87,500

by sveltejs · part of sveltejs/svelte

Investigate performance regressions and find opportunities for optimization

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🧰 Not standalone. This skill ships with sveltejs/svelte and only works together with that tool — install the tool first, then add this skill.

Investigate performance regressions and find opportunities for optimization

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This is the exact playbook injected into your agent when the skill activates — shown here so you can audit it before installing. You don't need to read it to use the skill.

name: performance-investigation description: Investigate performance regressions and find opportunities for optimization

Where outputs go

  • Summary report: benchmarking/compare/.results/report.txt
  • Raw benchmark numbers:
    • benchmarking/compare/.results/main.json
    • benchmarking/compare/.results/<your-branch>.json
  • CPU profiles (per benchmark, per branch):
    • benchmarking/compare/.profiles/main/*.cpuprofile
    • benchmarking/compare/.profiles/main/*.md
    • benchmarking/compare/.profiles/<your-branch>/*.cpuprofile
    • benchmarking/compare/.profiles/<your-branch>/*.md

The .md files are generated summaries of the CPU profile and are usually the fastest way to inspect hotspots.

Suggested investigation flow

  1. Open benchmarking/compare/.results/report.txt and identify largest regressions first.
  2. For each high-delta benchmark, compare:
    • benchmarking/compare/.profiles/main/<benchmark>.md
    • benchmarking/compare/.profiles/<branch>/<benchmark>.md
  3. Look for changes in self/inclusive hotspot share in runtime internals (runtime.js, reactivity/batch.js, reactivity/deriveds.js, reactivity/sources.js).
  4. Make one optimization change at a time, then re-run targeted benches before re-running full compare.

Fast benchmark loops

Run only selected reactivity benchmarks by substring:

Copy & paste — that's it
pnpm bench kairo_mux kairo_deep kairo_broad kairo_triangle
pnpm bench repeated_deps sbench_create_signals mol_owned

Tests to run after perf changes

Runtime reactivity regressions are most likely in runes runtime tests:

Copy & paste — that's it
pnpm test runtime-runes

Helpful script

For quick cpuprofile hotspot deltas between two branches:

Copy & paste — that's it
node benchmarking/compare/profile-diff.mjs kairo_mux_owned main foo

This prints top function sample-share deltas for the selected benchmark.

Practical gotchas

  • bench:compare checks out branches while running. Avoid uncommitted changes (or stash them) so branch switching is safe.
  • Each bench:compare run rewrites benchmarking/compare/.results and benchmarking/compare/.profiles.