Overview

When we first started working with the new parserOptions.projectService (formerly parserOptions.EXPERIMENTAL_useProjectService), it outperformed equivalent parserOptions.project setups by ~10-15%. We still sometimes have users report to us that it improved their linting speed. trpc/trpc#5868 is an example of one observation I've made.

However, at some point since then, we've started to notice worse performance of parserOptions.projectService in our own measurements. https://github.com/typescript-eslint/performance contains tooling to test this. From https://github.com/typescript-eslint/performance/blob/45f77c00c6d4384d466c39e8eb633a76f33dd019/README.md#results:

┌───────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┐
│ files │ project (even)       │ project (references) │ service (even)       │ service (references) │
┼───────┼──────────────────────┼──────────────────────┼──────────────────────┼──────────────────────┤
│ 128   │ '1.149 s ±  0.030 s' │ '1.135 s ±  0.008 s' │ '1.178 s ±  0.010 s' │ '1.736 s ±  0.012 s' │
│ 512   │ '1.636 s ±  0.009 s' │ '1.656 s ±  0.004 s' │ '1.895 s ±  0.007 s' │ '2.613 s ±  0.020 s' │
│ 1024  │ '2.353 s ±  0.013 s' │ '2.399 s ±  0.016 s' │ '3.130 s ±  0.017 s' │ '4.034 s ±  0.061 s' │
┴───────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┘

Summarizing those results, there seem to be two areas of slowness. Comparing project to projectService at ~1024 files:

• Overhead per-file ((even)), jumping from ~2.35s to ~3.13s
• Overhead switching to project references ((references)): jumping from ~2.4s to ~4.0s

⚠️ These measurements might be flawed. We are not sure there's not some external data polluting the results. This issue might just be tracking that we're measuring them incorrectly.

Also note that parserOptions.project has been in production for years, with many performance optimizations around it. parserOptions.projectService is very new and not yet stable. There is likely much more "low-hanging fruit" to optimize around parserOptions.projectService.

We don't yet know whether this is an issue in TypeScript or typescript-eslint. It's probably typescript-eslint.

Relevant past issues:

• Enhancement: Default allowAutomaticSingleRunInference to true #8121
• Bug: ~1.5x slowdown in Sentry codebase from project service #8424
• Bug: Performance regression from #7752 in project service due to changed file name casing #8519
• Bug: using with typescript v5.5 slows down linting by 2.3x #9474 -> perf: Node.getChildren speed regression in ts 5.5 when used at very large scale microsoft/TypeScript#59101
  • Note: the performance issues still show with TypeScript 5.4.5.