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docs: add Troubleshooting entry for debugging slow lint rules #9892

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18 changes: 16 additions & 2 deletions docs/troubleshooting/typed-linting/Performance.mdx
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Expand Up @@ -7,9 +7,23 @@ title: Performance
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As mentioned in the [type-aware linting doc](../../getting-started/Typed_Linting.mdx), if you're using type-aware linting, your lint times should be roughly the same as your build times.
As mentioned in [Linting with Type Information](../../getting-started/Typed_Linting.mdx), if you're using type-aware linting, your lint times should be roughly the same as your build times.
Most performance slowdowns in ESLint rules are from type-aware lint rules calling to TypeScript's type checking APIs.

If you're experiencing times much slower than that, then there are a few common culprits.
If you're experiencing lint times much slower than type-checking times, then there are a few common culprits.

## Slow ESLint Rules

ESLint includes a `TIMING=1` option documented in [Profile Rule Performance](https://eslint.org/docs/latest/extend/custom-rules#profile-rule-performance) that give a high-level overview of rule speeds.
However, because TypeScript utilizes internal caching, a project's _first type-aware lint rule will almost always seem the slowest_.

When investigating which lint rules are the slowest in your project, be sure to run them one at a time and compare those timing measurements separately.

To enable more complete verbose logging, you can use any of:

- [`eslint --debug`](https://eslint.org/docs/latest/use/command-line-interface#--debug): to enable all of ESLint's debug logs on the CLI
- [`parserOptions.debugLevel`](https://github.com/typescript-eslint/typescript-eslint/blob/7ddadda10845bc53967eeec83ba6b7cdc71a079f/packages/typescript-estree/src/parser-options.ts#L36): a shortcut to set `eslint`, `typescript`, and/or `typescript-eslint`
- Directly setting the `DEBUG` environment variable for [`debug`](https://github.com/debug-js/debug): e.g. `DEBUG=typescript-eslint:* eslint`

## Slow TypeScript Types

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