docs(perf): compare main engines and set next optimization targets

[huacnlee]

Issue # (if available)

N/A

Description of changes

Archive a fresh comparison of repository-pinned QuickJS-ng, quickjs-jit automatic mode, and Bun 1.4.0 default configuration after #19 and #20 merged into clean main 47aeb11. Include forced Tier 1/2 diagnostics, raw samples, methodology/provenance, CSV, confidence intervals, charts, and an evidence-based next-stage optimization roadmap. No runtime behavior changes.

Automatic JIT is faster than the interpreter in 8 workloads, statistically tied in 1, and slower in 13, based on per-workload paired 95% intervals. Selected results below express speed relative to the QuickJS-ng interpreter (1x = parity):

Workload	JIT automatic speed [95% CI]	Bun default speed
scalar-loop	24.024x [23.382, 24.316]	47.26x
float64-dense	7.819x [7.708, 7.895]	6.27x
property-heavy	0.268x [0.267, 0.271]	3.87x
generic-call-entry	0.135x [0.134, 0.136]	57.85x
arrays-typed	0.656x [0.654, 0.658]	10.41x

These are current-protocol results, not equally warmed peak-engine measurements: QuickJS and Bun differ in process-internal warmup and host timing boundaries. Native counters include warmup. Bun uses a recorded launcher that removes the runner's --smol flag. The unmerged property candidate 9ccda01 is excluded.

The roadmap prioritizes aligned measurement and steady-state counters, property and generic-call costs, automatic tiering/fallback overhead, then cold start and library hotspots. Property and generic-call stages propose a paired speed lower bound of 1.5x versus the same-harness main baseline, with eventual interpreter parity or better.

Evidence: full comparison, raw samples, methodology, next targets.

Validation:

• Completed 22 workloads × 5 modes × 30 latency samples, with 5 discarded warmup processes and 10 one-second throughput windows per workload/mode.
• Audited all 3,300 latency samples, checksums, entry/exit counters, source/binary hashes, derived results, links, and chart rendering; independent report review found no substantive issue.
• Staged whitespace check passes; CSV/SVG formatting normalization preserves data/path tokens. No runtime tests rerun for this documentation/data-only commit.
• Performance acceptance remains failing (runner exit 2 after complete output): compute speed lower bound is below 5x, automatic decision evidence is missing, and startup/definition-eval proxy budgets fail. The gpui-shell gate is inconclusive because external host evidence is absent. Raw gate output is preserved.

Checklist

• Changelog assessed: not applicable to benchmark evidence and planning documents.
• Unit tests assessed: no runtime changes; artifact and data validation completed.