> Emacs built with pgtk. Editing in f90 mode, though I don't think this is
> important. Display is WSLg. Severe input lag, to the point off becoming
> unusable, gets worse with more frames (or just time?). Heavy CPU usage by
> emacs while doing anything. Problem not seen when not built with pgtk.

I'm observing a similar high CPU usage problem with Emacs 29.1 on WSL1 and X server, not with WSLg.
As suggested by https://www.reddit.com/r/emacs/comments/urivlg/high_cpu_usage_in_wsl_with_emacs_29050/
reverting https://github.com/emacs-mirror/emacs/commit/fa52782f5c4eaef7138534766dfc8a29465785b2 solves the problem for me.