A possible explanation. Recorder chokes when I try to record a very large window (about 3000x1700 pixels – about 80% of my 4K monitor). If I shrink the window down to 1300x900, Recorder seems much happier. So this might be a simple “too many pixels to record in real time” problem?
(while my Rack and hardware config hasn’t changed, I did recently change my external monitor for a gargantuan 4K monster and started running Rack in much larger windows. So this may be the root cause of the problem. I’ll check in with VCV support to see if they’ve done any testing on large windows.