There's no definitive performance test for ESLint v8 vs. v9 because it depends largely on what you have in your configuration file.

When ESLint v9.0.0 was released, there were some performance regressions, and those are all fixed now.

In general, people seem to get roughly the same performance out of v9 that they did in v8. If you find a big difference, please let us know.

FWIW, I upgraded a project few days ago, and the lint step got 2x faster. Seems quite unbeliavable, so I'm still a bit paranoid on whether I missed something when migrating the config, but so far everything seems to be working correctly. Needless to say there's also typescript-eslint and eslint-plugin-vue at play, so that might change things quite a bit compared to vanilla eslint.

For my medium-sized project (~250 kB of .js and .vue files), ESLint 9 is generally similar in performance to 8. I'm using --cache, and both versions run for around 3-5 seconds depending on the number of changed files.

However, that's only in the general case. Because randomly, several times a day, without any apparent reason, ESLint 9 suddenly takes 15-20 seconds instead of the usual 3-5. I may have switched code branches or changed configuration in the meantime, but I can never reproduce this effect doing the same on purpose afterwards, and I don't see a reason why ESLint should ever take longer than it normally does without cache. I've never seen such massive performance differences with ESLint 8, or with any other tool really.

I started running it with --debug, and I noticed that when the slowdown happens, all the debug lines appear as if in slow motion. Here are some example lines for comparison:

Normal run:

  eslint:rules Loading rule 'no-empty-pattern' (remaining=269) +18ms
  eslint:rules Loading rule 'no-extra-parens' (remaining=268) +6ms
  eslint:rules Loading rule 'no-loss-of-precision' (remaining=267) +6ms
  eslint:rules Loading rule 'no-restricted-syntax' (remaining=266) +17ms
  eslint:rules Loading rule 'no-sparse-arrays' (remaining=265) +7ms
  eslint:rules Loading rule 'no-useless-concat' (remaining=264) +41ms
  eslint:rules Loading rule 'object-curly-newline' (remaining=263) +8ms
  [...]
  eslint:eslint 108 files found in: 2097ms +2s

Slow run:

  eslint:rules Loading rule 'no-empty-pattern' (remaining=269) +129ms
  eslint:rules Loading rule 'no-extra-parens' (remaining=268) +16ms
  eslint:rules Loading rule 'no-loss-of-precision' (remaining=267) +28ms
  eslint:rules Loading rule 'no-restricted-syntax' (remaining=266) +110ms
  eslint:rules Loading rule 'no-sparse-arrays' (remaining=265) +45ms
  eslint:rules Loading rule 'no-useless-concat' (remaining=264) +308ms
  eslint:rules Loading rule 'object-curly-newline' (remaining=263) +54ms
  [...]
  eslint:eslint 108 files found in: 8997ms +9s

Since it's very annoying when routine linting takes several times longer than usual, I'm downgrading to ESLint 8 for now.

In our experience, when we migrated our large (many plug-ins/rules) eslint config from v8 to v9, the startup of eslint got significantly slow. I haven't captured before-and-after figures, but the difference is felt across the entire development team. When running with --debug option, the logs appear to pause for a good number of seconds in the config-loader initial phase, right before it starts loading the rules. While it wasn't exactly fast with v8, It wasn't that bad either.