MacBook Air M1, 16Gb RAM. Opening TB takes <2s and everything is snappy. Searches are very fast. I still find the UI and UX a bit clunky but performance has not been an issue for me.

Doing this will speed it up: https://support.mozilla.org/en-US/kb/rebuilding-global-datab...

i9 with 64gb of ram and a Samsung pro SSD. I have no issues. Either your mailbox is substantially bigger or your database is broken.

I have the 16years old Gmail in it, so that must be the cause. Should I just delete the .db and let it rebuild? Would that fix/mitigate the issue?

I don’t know, please backup .Mozilla or whatever path it sits in before you experiment.

Another use in this thread suggested deleting .db, but ymmv