@a6ff0b2b ‼️ Another piece of information: I don't believe it's AppKit's window snapping at all, because if you hold the option key to disable snapping you still get the window drag lag (just without the jitter).

I have toggled all the mouse preferences I can find like acceleration et al and it makes no difference 

 @9c59d669 I'm not sure if the fact that holding down Option stops the jitter, leaving only the lag, exonerates that part of the windowing system or implicates it!