pcbjam/docs/features/perf

Cross-browser performance: why Firefox > Chrome > Safari, and how to close the gap

Research notes, 2026-06-18. The KiCad WASM port runs fastest in Firefox, slower in Chrome, slowest in Safari. This document explains why at the browser-engine level and lays out a ranked, build-specific plan to speed up Chrome and Safari. Web claims are dated and linked in Sources; codebase claims carry file:line refs. Companion work lives in ../async/ (Asyncify) and ../wasm-exceptions/ (the -fwasm-exceptions migration).

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TL;DR

The Firefox lead is not a Firefox trick. Our binary is dominated by Asyncify instrumentation, and Firefox's compilers simply tolerate Asyncify's pathological code far better than Chrome's or Safari's do. So the highest-leverage work for Chrome and Safari is to shrink/attack the Asyncify footprint, plus a handful of cheap, orthogonal wins.

There are two independent axes, and both need attention:

1. WASM compile/execute — Asyncify-dominated. This explains the Firefox > Chrome > Safari ordering.
2. WebGL rendering — Safari's Metal/ANGLE overhead. This is extra Safari slowness on top of axis 1, and several fixes are one-liners.

Ranked levers

#	Lever	Axis	Effort	Impact	Where
1	Confirm/force instantiateStreaming + Content-Type: application/wasm + stable URL/ETag	startup	hours	~1.5–1.8× cold start (FF); arms V8 cache	web/standalone/src/wasm/boot.ts
2	Brotli instead of gzip-9 on R2	startup	hours	~15–25% smaller transfer	R2 / edge config
3	powerPreference: 'high-performance' + context-lost handlers	WebGL (Safari/Chrome)	hours	discrete GPU instead of integrated	wxwidgets/src/wasm/glcanvas.cpp:524-535
4	Audit GAL shaders for the flat qualifier	WebGL (Safari)	hours–days	up to seconds/frame in worst case	kicad/common/gal/shaders/
5	Remove glGetError() from the render loop	WebGL (Safari)	hours	avoids per-call Metal flush	GAL compositor
6	Test antialias: false	WebGL (Safari)	hours	cuts MSAA resolve cost	glcanvas.cpp:524-535
7	Enable -msimd128	WASM exec (all)	days	1.5–2.5× geometry/render hot loops	build flags
8	ASYNCIFY_ADVISE → ASYNCIFY_IGNORE_INDIRECT + extend REMOVE	WASM exec (all, esp. Chrome/Safari)	days	smaller binary + faster tier-up	scripts/common/apply-asyncify.sh
9	-fwasm-exceptions (size)	WASM (all)	weeks	64.5 → 36 MB gz	tracked — see § Structural bets
10	JSPI (delete Asyncify)	WASM (all, esp. Safari)	weeks	~40–50% smaller, removes JIT pressure	tracked — see § Structural bets
—	wasm-split, WebGPU GAL backend	startup / WebGL	weeks+	deferred (see § Deferred)	—

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Current build (the baseline)

Verified from the build scripts and runtime glue:

Knob	Value	Location
Asyncify	-sASYNCIFY=1, ASYNCIFY_STACK_SIZE=65536	scripts/kicad/build-kicad-target.sh:~400
Exceptions	legacy -fexceptions (not -fwasm-exceptions)	build-kicad-target.sh:240-255
SIMD	none (-msimd128 absent)	—
Threads	-sUSE_PTHREADS=1, pool = navigator.hardwareConcurrency (+ COOP/COEP)	build-kicad-target.sh, web/.../preflight/capabilities.ts
Memory	INITIAL_MEMORY=256MB, MAXIMUM_MEMORY=4GB, ALLOW_MEMORY_GROWTH=1	build-kicad-target.sh
Opt	clang -O2 (release); link -O0 then host wasm-opt -O2 after --asyncify	apply-asyncify.sh:88-157
WebGL	WebGL2 (-sMAX_WEBGL_VERSION=2), antialias:true, powerPreference:DEFAULT	glcanvas.cpp:524-535
Loading	Emscripten script-glue; streaming not confirmed; gzip-9, no Brotli	boot.ts:145-294
Artifact	pcbnew 186 MB raw / 64 MB gzip; eeschema 99/34; pl_editor 52/17; gerbview 49/16	output/

Note: all three modern browsers support SharedArrayBuffer/threads under COOP+COEP (the app demonstrably runs in each) — capability gating is in capabilities.ts, not UA sniffing.

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Why the ordering exists (engine internals)

The villain: Asyncify

Asyncify rewrites every instrumented function with unwind/rewind state checks and saves/restores all locals to linear memory. That expands each local's live range across the whole function, producing a nearly fully-connected interference graph — exactly the input that is catastrophic for optimizing register allocators. Asyncify's own docs warn: "VMs may also limit compilation to the baseline tier on such pathological code." Result: ~+70% binary, giant functions, and the 186 MB-raw pcbnew. See ../async/02-asyncify-internals.md.

How each engine copes

Engine	Baseline tier	Optimizing tier	On Asyncify's giant functions
Firefox / SpiderMonkey	Rabaldr, ~25 ns/byte, eager whole-module, multithreaded (30–60 MB/s)	Ion — 75× large-function fix, Oct 2024 (sorted live ranges, Semi-NCA dominators, sparse bitsets) targeting exactly the huge-CFG/high-vreg shape Asyncify creates (ONNX: 5 min → 3.9 s)	Best. Whole module baseline-compiled before download finishes; Ion swallows the big functions. No OSR gap.
Chrome / V8	Liftoff, ~50 ns/byte (½ Firefox)	TurboFan — chokes on huge fns (a 1.96 MB fn → 95 s, 7.4 GB RAM, 87% in regalloc); falls back to mid-tier allocator or skips optimization	Middle. V8 has no OSR for wasm — a function in a long loop (Asyncify rewind/unwind loops!) finishes that whole call in Liftoff; only the next call gets TurboFan.
Safari / JSC	Lazy everything: IPInt (interpreter) → BBQ → OMG. Nothing eager.	OMG (B3) — did not get Ion's 2024 large-fn treatment	Worst. First run executes at interpreter speed; Asyncify ~doubles fn count → huge OMG backlog → documented 300–400% CPU spike for 30 s+ after a workload. No persistent compiled-code cache, so it re-pays every session; above ~10 MB it switches to a slower JIT mode.

Two corollaries that bite us specifically

• Chrome's V8 wasm code cache is effectively unavailable. It only caches modules under ~150 MB compiled, and compiled code is 5–7× the .wasm. Our 186 MB pcbnew → ~1 GB compiled — far over the ceiling. So Chrome re-runs TurboFan on every cold load today. Shrinking the binary (levers 7–10) is the only way to get Chrome's repeat-load cache back. See V8 wasm code caching.
• Benchmark trap: with DevTools open, V8 tiers all wasm down to Liftoff. Never measure Chrome speed with DevTools open (except via an actual Performance recording, which forces tier-up). This likely makes Chrome look worse than it is in casual testing.

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The ranked plan

Tier 1 — cheap, do now (days, low risk)

1. Confirm + force streaming instantiation and cache headers. The loader injects the Emscripten JS glue via <script> (boot.ts); whether the runtime then streams the .wasm depends on serving conditions. In DevTools → Network, confirm the .wasm returns Content-Type: application/wasm with no console "falling back to ArrayBuffer instantiation" warning. Streaming is ~1.5–1.8× faster cold-start on Firefox and is the only path that arms V8's code cache. Serve the .wasm from a stable URL (no content-hash in the path; use a stable alias) with ETag/304. With -pthread, confirm the module is compiled once and shared to workers (Emscripten does this via the shared WebAssembly.Module), not recompiled per worker.

2. Brotli instead of gzip-9 on R2. Brotli is ~15–25% smaller on wasm (our 64 MB pcbnew → ~50 MB). Verify Cloudflare actually Brotli-compresses it at the edge — it often skips large binary types — and if not, precompress and serve with Content-Encoding: br + Content-Type: application/wasm. Smaller transfer also means less to compile, so it compounds with everything below.

3. powerPreference: 'high-performance' for WebGL. We default to EM_WEBGL_POWER_PREFERENCE_DEFAULT (glcanvas.cpp:524-535). Safari (and Chrome on dual-GPU Macs) defaults WebGL to the integrated GPU. Requesting high-performance switches to the discrete GPU — often the single biggest GPU-bound framerate win on MacBook Pros. Caveat: Safari only honors it if you also register webglcontextlost/webglcontextrestored handlers.

4. Audit GAL shaders for the flat interpolation qualifier. This is the big Safari sleeper. flat triggers a provoking-vertex workaround in Safari's Metal/ANGLE backend that has cost real apps seconds per frame. PCB renderers commonly use flat for per-primitive net/layer colors. Grep the GAL shaders (kicad/common/gal/shaders/, source GLSL 1.20 before convert_glsl_es3.py); if present, replace with regular interpolation or restructure. Potentially a massive Safari-only win.

5. Remove glGetError() from the render loop. On Safari each call forces a Metal pipeline flush. Restrict to init/debug builds only. (Note: the WebGL compositor already drains stale glGetError() once before draws — that's fine; the concern is per-call error checks inside the hot path.)

6. Test antialias: false. We default MSAA on (antialias:true). KiCad's GAL does much of its line AA in-shader (SMAA) and has its own AA setting; if MSAA is redundant, dropping it cuts Metal's resolve cost on Safari. Quality/perf tradeoff — A/B it on a dense board; consider exposing it as a setting.

Tier 2 — medium effort, high impact

7. Enable -msimd128. Expect 1.5–2.5× on the geometry/render hot loops (polygon booleans in shape_poly_set, DRC overlap checks, vertex-buffer fills) via LLVM autovectorization at -O2+. Safe on all three engines (Chrome 91 / FF 89 / Safari 16.4). Helps absolute Chrome and Safari speed. Verify v128.* actually appears in the disassembly for the hot functions, and prefer pmin/pmax over min/max (the SSE→wasm emulation table has slow paths). Do not ship Relaxed SIMD yet (Safari still flags it). Minor interaction to watch: SIMD slightly grows per-function size, which feeds the Asyncify/locals pressure — measure after the wasm-opt -O2 pass.

8. Shrink the Asyncify surface. This directly attacks the root cause for Chrome and Safari. Run ASYNCIFY_ADVISE to see which functions get instrumented and why — it surfaces the biggest instrumented functions (the JIT pressure points). Then:

• ASYNCIFY_IGNORE_INDIRECT=1 is the high-impact one: our wxWidgets/GAL code is vtable-heavy, and Asyncify conservatively instruments every indirect call site, which is why instrumentation spreads everywhere. With our understanding of the suspend paths (the park-throw work), we may be able to assert no indirect call is on the suspend stack and add specific ones back via ASYNCIFY_ADD.
• Extend the existing 12-function ASYNCIFY_REMOVE list (apply-asyncify.sh:92-104) with cold/startup-only large functions ADVISE flags.
• Smaller instrumented set → smaller functions → better tier-up in all engines and a smaller Safari OMG backlog.
• ⚠️ Error-prone (wrong config = silent runtime breakage). Gate behind the red/green Asyncify harness in ../asyncify-arbiter/redgreen.md / tests/asyncify/.

Structural bets (track / prototype)

9. -fwasm-exceptions (we're on legacy -fexceptions). Biggest size lever — ../wasm-exceptions/02-measurements.md puts pcbnew at 64.5 → 36 MB gzip, which would also start bringing Chrome back under the code-cache ceiling and fix the unreliable catch/destructor landing-pad behavior we've documented. Two blockers to track before committing:

• a Safari 26.0 startup regression for -fwasm-exceptions modules (emscripten #25365) — verify whether it's fixed in a 26.x / Safari 27 beta before shipping, since Safari is the browser we're trying to help;
• the asyncify-EH unwind-from-catch interaction (../wasm-exceptions/05-asyncify-fork-design.md).

10. JSPI — the eventual real fix for Safari, because it deletes Asyncify entirely (no instrumentation → no giant functions → no OMG backlog → the 300–400% Safari spike goes away) and cuts ~40–50% of binary size. Status:

Engine	JSPI status
Chrome / V8	shipped, Chrome 137 (May 2025)
Firefox / SpiderMonkey	Firefox 153 intent-to-ship (June 2026); Nightly now, stable ~late summer/fall 2026
Safari / JSC	Safari 27 beta (WWDC26), enabled by default; stable Fall 2026

Don't migrate wholesale yet:

• unresolved ~350× regression on the JS→C→JS re-entry pattern (emscripten #21081) — exactly what a GUI event loop hits constantly;
• static-init SuspendError (emscripten #24302);
• invoke_* over-tagging under our legacy exceptions.

Recommended path: prototype JSPI on a small tool (calculator or pl_editor) behind feature detection ('Suspending' in WebAssembly) with Asyncify fallback, profile the re-entry pattern, and watch #21081. By the time it's safe, all three engines will support it.

Deferred / not now

• wasm-split / -sSPLIT_MODULE — the secondary module can't be loaded lazily and asynchronously on the main thread, which is incompatible with our main-thread Asyncify model unless we move to -sPROXY_TO_PTHREAD.
• WebGPU GAL backend — the structural exit from Safari's Metal/ANGLE overhead (Safari 26 ships WebGPU), but it's a multi-week GLSL→WGSL port with no upstream KiCad support. See the GAL history in ../archive/webgl/.
• Global -O3 — its inlining bloats the binary and makes the Chrome cache/compile problem worse. If anything, compile cold/utility units at -Os.

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What to do first

Quick, visible, near-zero-risk wins this week:

• Safari: #3 (high-performance GPU) + #4 (flat audit) + #5 (glGetError).
• Startup everywhere: #2 (Brotli) + #1 (streaming/headers).
• Runtime everywhere: #7 (-msimd128).

Then invest in #8 (Asyncify ADVISE + IGNORE_INDIRECT) as the real lever against the Chrome/Safari gap, and keep #9 / #10 on a tracking list.

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Sources

Engine internals

• 75× faster: optimizing the Ion compiler backend — SpiderMonkey, Oct 2024
• Understanding WebAssembly code generation throughput — wingolog, 2020
• V8 WebAssembly compilation pipeline · Dynamic tiering · Liftoff
• Code caching for WebAssembly developers — V8
• Introducing the JetStream 3 Benchmark Suite — WebKit, 2024 (IPInt/BBQ/OMG)
• Pause and Resume WebAssembly with Binaryen's Asyncify — kripken, 2019

Startup / size / SIMD

• Optimizing WebAssembly Startup Time — Nutrient
• MDN: WebAssembly.instantiateStreaming
• Module Splitting — Emscripten
• Using SIMD with WebAssembly — Emscripten · V8 SIMD · caniuse wasm-simd
• Asynchronous Code (Asyncify settings) — Emscripten

Safari WASM + WebGL

• WebGL Performance on Safari & Apple Vision Pro — Wonderland Engine (flat, UBO timing)
• WebKit features in Safari 26.0 · News from WWDC26 — Safari 27 beta
• emscripten #25365 — Safari 26.0 wasm-exceptions regression
• emscripten #26027 — Safari + Asyncify + unaligned-load leak
• ONNX Runtime #26827 — Safari WebKit 26 OMG CPU loop

JSPI

• V8: Introducing the WebAssembly JavaScript Promise Integration API · new API
• caniuse: JSPI · Chrome 137 release notes
• Mozilla dev-platform: Intent to Ship JSPI (Fx153), June 2026
• emscripten #21081 — JSPI 350× slower for JS→C→JS · #24302 — JSPI static-init SuspendError

Local cross-references

• ../async/ — Asyncify internals, single-slot contention, park-throw.
• ../wasm-exceptions/ — -fwasm-exceptions measurements, toolchain status, asyncify-EH fork design.
• ../asyncify-arbiter/redgreen.md — the harness to gate Asyncify-surface changes.
• ../archive/webgl/ — GAL → WebGL2 history (context attrs, compositor, shaders).
• Build: scripts/kicad/build-kicad-target.sh, scripts/common/apply-asyncify.sh.
• WebGL context: wxwidgets/src/wasm/glcanvas.cpp:489-535. Loader: web/standalone/src/wasm/boot.ts.