KM very slow and laggy after Sonoma upgrade

The window manipulation issue is a fairly well-documented one, dating back at least a couple of years. Sonoma seems to have made the matter worse however.

Anyhoo, note Peter's comments on one particular post:

Make sure you are up to date with the latest KM version as he implemented a workaround to mitigate the crappy Accessibility API (which is posted somewhere else on the forum but I can’t find it right now).