[Meads]

I've had these GUI lags on all kinds of computers and I have never been able to pinpoint it 100%. But it was never a CPU or other performance issue.
My recent Hackintosh - for whatever reason - is the first and only machine I've used where I didn't experience this lag. I wish I knew why, but unfortunately I don't. Nothing changed periphery or software wise.

However what I've noticed when those lags occured was the following:

There's a process running in the background of MacOS called "WindowServer". It handles all kinds of display activities. If you force quit it from activity monitor you're being kicked out of MacOS back to the login screen. BUT - it consistently solved the lag for a while. Just logged back in again and everything was snappy.
It's feels like some kind of graphical memory leak (don't quote me on that).
Give it a try, sure won't hurt anything.