I suppose I've not tried it in any sort of extreme scenario, but it's not clear to me what dired would be slow at. Functions like copy-file are implemented in C.

Hasn’t been a problem for me for years. It seems to me that often this argument is based (directly or indirectly) from conditions 20 or more years ago. I think Emacs itself hasn’t changed much (it’s gotten better, but not fundamentally different), but the supporting environments (read: hardware) easily make for a usable experience. There are better poster children for bloat these days.