Environment

Node version: 18.8.0
npm version: 8.19.1 (local and 8.18.0 global)
Local ESLint version: 8.23.1
Global ESLint version: -
Operating System: macOS 12.6
CPU: M1

What parser are you using?

Default (Espree) + jsonc-eslint-parser

What did you do?

I'm trying to migrate to the flat config.
Old config: https://github.com/zloirock/core-js/blob/master/.eslintrc.js
New config: https://github.com/zloirock/core-js/blob/eslint-flat/eslint.config.js

Just run npm i && npm run lint.

With the old-style config, lining (directly eslint) takes 32 seconds.
With the new-style config, lining (directly eslint) takes 2 minutes and 34 seconds.

What did you expect to happen?

The same (or better) performance.

What actually happened?

5x performance degradation.

Participation

• I am willing to submit a pull request for this issue.