I've only noticed this issue, slow to respond to keypresses in regular typing, when using a few (could not identify anymore) language modes. Some of them are really inefficient in the way they're written which shows up when you get a larger source file (not even large, I recall some mode creating problems with anything over about 300 lines of code which is not excessive for a single file in many languages). But normally, using emacs for {cpp, common lisp, erlang, elixir, ada, org, prolog, python}, I've not found it to be slow at all.

I'd dig into what modes (minor and major) are setup on the files you're editing.

I will also say, there was some way I'd once configured org-mode that really slowed it down when there were a lot of code blocks (as it could switch into the appropriate major mode for that block). I don't recall ever "fixing" it, it just got faster over time. I was specifically using org for literate programming. When using it straight with primarily the outliner, dates, todo lists, etc. it didn't have the same issue.