I “live in Emacs”, like most of us (atleast at work). It has been getting slower release on release, async support doesn’t seem to have been picked up by most packages and native-comp has made it more brittle.
Over this time, we moved from running the OS on hardware to running in VMs, so fractional slowdown was expected. But what I have is a few X slower Emacs. I had never seen Emacs take a minute to indent a few thousand lines, for example. Maybe some modes have slower code.
Emacs doesn’t seem slower for me than in bygone years. If anything, recent versions are faster because of native Lisp compilation. But, some language modes did slow down a lot (while gaining generality) as they switched from custom parsers to LSP, tree-sitter, and so on. Maybe that’s what you’re seeing.
I did some comparisions. The installation built without treesitter support is noticeably faster than the one built with treesitter support; even in the latter, I don’t have any treesitter grammars at all and no -ts-mode in use.