|I generally run into the slowdown maybe once every half-hour during real-world usage; a common trigger seems to be mouse clicks, drags, or scrolls, though I haven’t been able to pin down any predictable repro. cases aside from the shake-the-divider one.|
As a janky workaround, I’ve found that switching to a different desktop and back during a slow redraw allows all the buffered draws to go through instantly; not sure if that implies anything useful.
The "shake the divider" case becomes much worse, lots of flickering and incomplete text.
Wheter it prevents the slow redraws in real usage is hard to tell. It is not easily reproduced in normal Emacs usage.