Limit default number of JS workers to 4

[devongovett]

This limits the default number of JS worker threads to 4, even if there are more CPUs available. Now that we run the JS transformer and resolver in native Rust threads (#9147), the JS threads don't have as much to do. Benchmarking indicates that > 4 JS threads has negligible to negative performance impact, and limiting the number reduces memory usage a lot.

The PARCEL_WORKERS environment variable can still be set to override this if the default doesn't work for a particular project.

Closes #5617. Closes #5072.

[parcel-benchmark]

Benchmark Results

Kitchen Sink ✅

Timings

Description	Time	Difference
Cold	1.47s	+28.00ms
Cached	296.00ms	+47.00ms ⚠️

Cold Bundles

Bundle	Size	Difference	Time	Difference
dist/legacy/index.html	826.00b	+0.00b	389.00ms	+26.00ms ⚠️
dist/modern/index.html	749.00b	+0.00b	389.00ms	+26.00ms ⚠️
dist/legacy/index.b8ae99ba.css	94.00b	+0.00b	269.00ms	+18.00ms ⚠️
dist/modern/index.31cedca9.css	94.00b	+0.00b	269.00ms	+18.00ms ⚠️

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/legacy/index.b8ae99ba.css	94.00b	+0.00b	252.00ms	-17.00ms 🚀
dist/modern/index.31cedca9.css	94.00b	+0.00b	252.00ms	-17.00ms 🚀

React HackerNews ✅

Timings

Description	Time	Difference
Cold	3.88s	+16.00ms
Cached	415.00ms	+2.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/logo.8dd07848.png	244.00b	+0.00b	240.00ms	+16.00ms ⚠️

AtlasKit Editor ✅

Timings

Description	Time	Difference
Cold	31.61s	-361.00ms
Cached	2.13s	+3.00ms

Cold Bundles

Bundle	Size	Difference	Time	Difference
dist/refractor.3e0cc31b.js	598.96kb	+0.00b	10.16s	-592.00ms 🚀
dist/media-viewer.38e3999a.js	536.13kb	+0.00b	10.17s	+2.38s ⚠️
dist/popup.a77286c1.js	321.45kb	+0.00b	10.16s	-590.00ms 🚀
dist/ConfigPanelFieldsLoader.182d39bc.js	303.43kb	+0.00b	7.05s	-752.00ms 🚀
dist/EmojiPickerComponent.4a196252.js	188.61kb	+0.00b	10.14s	-584.00ms 🚀
dist/card.d06de810.js	138.91kb	+0.00b	7.04s	-748.00ms 🚀
dist/ConfigPanelFieldsLoader.28b428a5.js	82.73kb	+0.00b	10.14s	-588.00ms 🚀
dist/esm.34897092.js	62.95kb	+0.00b	10.16s	-591.00ms 🚀
dist/ElementBrowser.e8f01080.js	61.94kb	+0.00b	7.04s	-753.00ms 🚀
dist/archive.c374f622.js	59.90kb	+0.00b	10.17s	+2.38s ⚠️
dist/esm.bfca2115.js	59.30kb	+0.00b	7.05s	-747.00ms 🚀
dist/component-lazy.51d1dec9.js	58.94kb	+0.00b	5.07s	+310.00ms ⚠️
dist/esm.5e913efb.js	39.11kb	+0.00b	10.16s	-590.00ms 🚀
dist/smartMediaEditor.efa59853.js	21.68kb	+0.00b	10.17s	-581.00ms 🚀
dist/esm.aee9cbf1.js	20.43kb	+0.00b	10.17s	-581.00ms 🚀
dist/ConfigPanelFieldsLoader.2b7c03be.js	15.74kb	+0.00b	7.04s	-755.00ms 🚀
dist/ui.8c117104.js	14.48kb	+0.00b	7.04s	-749.00ms 🚀
dist/ConfigPanelFieldsLoader.5dfde67d.js	13.63kb	+0.00b	7.04s	-754.00ms 🚀
dist/dropzone.77a8e729.js	13.40kb	+0.00b	10.16s	-591.00ms 🚀
dist/pdfRenderer.01deafa1.js	12.04kb	+0.00b	7.04s	-750.00ms 🚀
dist/dropzone.1c15cdc1.js	11.48kb	+0.00b	10.16s	-591.00ms 🚀
dist/Toolbar.4d256e97.js	9.36kb	+0.00b	10.17s	-581.00ms 🚀
dist/clipboard.400013a2.js	7.92kb	+0.00b	10.16s	-591.00ms 🚀
dist/mobile-upload.3baad8e4.js	7.79kb	+0.00b	7.05s	-747.00ms 🚀
dist/mobile-upload.7a892a37.js	7.79kb	+0.00b	7.05s	-748.00ms 🚀
dist/mobile-upload.2102debb.js	7.79kb	+0.00b	10.17s	-581.00ms 🚀
dist/index.runtime.e32c1d2d.js	7.29kb	+0.00b	10.19s	-569.00ms 🚀
dist/browser.0009c8b4.js	7.19kb	+0.00b	10.16s	-589.00ms 🚀
dist/index.b16227d6.css	4.08kb	+0.00b	10.19s	-619.00ms 🚀
dist/media-viewer-analytics-error-boundary.60bdaa4c.js	3.18kb	+0.00b	10.17s	-582.00ms 🚀
dist/media-picker-analytics-error-boundary.c493f011.js	3.18kb	+0.00b	10.16s	-589.00ms 🚀
dist/media-card-analytics-error-boundary.74e0c7f9.js	3.18kb	+0.00b	10.17s	-582.00ms 🚀
dist/ru.0cf3f40e.js	2.81kb	+0.00b	7.04s	-754.00ms 🚀
dist/uk.282f23b1.js	2.76kb	+0.00b	7.04s	-755.00ms 🚀
dist/codeViewerRenderer.51140ec8.js	2.61kb	+0.00b	10.17s	+2.38s ⚠️
dist/th.137e1013.js	2.60kb	+0.00b	7.04s	-754.00ms 🚀
dist/vi.b46097db.js	2.09kb	+0.00b	7.04s	-755.00ms 🚀
dist/tr.c85d90a9.js	2.03kb	+0.00b	7.04s	-754.00ms 🚀
dist/sv.1c06c95c.js	1.98kb	+0.00b	7.04s	-754.00ms 🚀
dist/zh_TW.b7c55aa6.js	1.86kb	+0.00b	7.04s	-755.00ms 🚀
dist/zh.b01fe721.js	1.84kb	+0.00b	7.04s	-755.00ms 🚀
dist/workerHasher.540c9790.js	1.56kb	+0.00b	7.04s	-753.00ms 🚀
dist/workerHasher.c840c607.js	1.56kb	+0.00b	7.05s	-747.00ms 🚀
dist/workerHasher.730f3766.js	1.56kb	+0.00b	10.16s	-591.00ms 🚀
dist/workerHasher.9b1fcdbf.js	1.56kb	+0.00b	10.16s	-589.00ms 🚀
dist/workerHasher.02b63a21.js	1.56kb	+0.00b	10.17s	-581.00ms 🚀
dist/sk.4be9c93f.js	656.00b	+0.00b	7.04s	-754.00ms 🚀
dist/simpleHasher.c14e20b4.js	589.00b	+0.00b	7.05s	-747.00ms 🚀
dist/simpleHasher.23db7a52.js	589.00b	+0.00b	7.05s	-747.00ms 🚀
dist/simpleHasher.eefc98b4.js	589.00b	+0.00b	10.16s	-591.00ms 🚀
dist/simpleHasher.47b9c809.js	589.00b	+0.00b	10.16s	-590.00ms 🚀
dist/simpleHasher.cadc19c6.js	589.00b	+0.00b	10.17s	-581.00ms 🚀
dist/ro.8d5b380a.js	482.00b	+0.00b	7.04s	+1.75s ⚠️
dist/index.html	248.00b	+0.00b	10.20s	+4.90s ⚠️

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/media-viewer.38e3999a.js	536.13kb	+0.00b	7.49s	-2.85s 🚀
dist/component-lazy.51d1dec9.js	58.94kb	+0.00b	4.69s	-534.00ms 🚀
dist/ro.8d5b380a.js	482.00b	+0.00b	5.25s	-1.91s 🚀
dist/index.html	248.00b	+0.00b	10.57s	+5.33s ⚠️

Three.js ✅

Timings

Description	Time	Difference
Cold	2.85s	+26.00ms
Cached	318.00ms	+8.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

Click here to view a detailed benchmark overview.