> I've never seen that [memory pressure] halt animations, mouse motion, or stop the clock from updating.
This is exactly what I've seen. If I catch it early enough and can get the UI to respond to me tabbing to the terminal, I can kill Safari's web content processes or whatever it is that's leaking memory and it goes away.
The only thing I've seen stop the clock (besides SystemUIServer itself hanging) is a forkbomb. But even that didn't stop the cursor. If the cursor is stopping, that indicates something more serious is going on.
I do agree that Apple needs to get better at I/O scheduling – even with an SSD the experience is less responsive than BeOS was on 90s PC hardware — but the animation loop has typically been a reliable diagnostic cue in my experience for distinguishing between swapping / disk failure and other issues.