It's linting the entire CPython codebase and standard library, which is quite large (possibly one of the largest python codebases). But yes, pure python linters will be inherently quite a bit slower than a native implementation. The point of ruff is that it's so fast you forget or never even notice it's running. Every change, every little modification, etc. should always be linted with near instant feedback.

I worked on a 50k-100k loc python codebase where the linters took a full minute to run if you deleted your cache. And I did try to optimize it. I wouldn't be surprised if that's normal.

If you have to do things like parsing or AST walking a lot, you run into the slow performance of the core bytecode interpreter pretty quickly. For Python, this is a compute-heavy workload.