I think that it still struggles a bit when I have many (>100) buffers open, largely I think because of anything that has to do window management winding up iterating over every buffer, and it turns out a lot of things (both internal and third-party packages) wind up doing this on various hooks that make the overall experience feel sluggish.

I also still see occasional performance issues with eglot/LSP on very large projects, especially around input feeling laggy while the language server is churning on something.

In general, it's frustrating when stuff that /should/ be in the background and not have any effect on actual editing latency winds up causing slowness or hangs. A good example is any time font-locking gets complicated, e.g. on very large files (which again has improved in recent versions).