Hello!
I had some back and forth with Canvas on this. I sent in images and specific assignments when it showed up. The case was closed when I didn't respond during the hurricane. When it starts happening again, I think I'll try again.
They noted that the slow loading was a result of students navigating away from the page too early when submitting and a preview image wasn't generated.
They couldn't recreate any of the other issues (generally, it's pretty random across many different assignments). I saw no pattern other than all the assignments I had trouble with were using the Turnitin LTI.
So, no solution currently.
I'm glad it's not just me (or my institution) seeing this. Best of luck!
best,
Matt