A recent upgrade of CODE in last day or so is enormously faster for me, I am using a nightly build 6.4.8 (git hash: [6a844e4] (javascript:void(window.open(‘Commits · CollaboraOnline/online · GitHub’));)).
The improvements I see may be to do with the performance wins mentioned in Collabora Online Weekly Meeting #23 so try 6.4.8*4 or later.
My gut feeling is recent releases have been slower than normal and this release has tuned up the performance of new software that was introduced (Toolbars and Canvas?). I wouldn’t have a clue if this will help you. So this is just fyi.
btw a week or 2 ago I upgraded to Nextcloud v21 in another environment and noticed an improvement over v20, but that could be my shared cloud server!
It is back to performing just about like a local app for me, and I am another continent away from my server.