Karina,
I can't recall the finer details. I made the move to the new hub maybe two weeks before it was to be done for me (scary thought so I tried it myself).
For about two weeks, my older files opened very slowly, if at all. Many times, I would just kill Fusion and try again. I have an impression that once opened the file would be better or even normal the next time.
I have just opened several old files that I probably did not try during the troubled time. They open, not as quickly as I'd expect if there was local storage, but not as bad as I've seen.
I don't see an option for "add to local cache", perhaps that is the same as "add to offline cache".
I tried that a couple of times with no problems; switched to "offline" mode and opened the file again. Maybe it was faster--I didn't time it carefully.
BUT, when I switched back to "online" mode, I got:

The dashboard says

the history goes back 3 hours. I wonder if this effect is related to our problems.