I've noticed this issue since the last release as well. Specifically, even if I restart the browser, unload/reload the tab, etc., Fastmail is immediately slow when it comes up. It also isn't just slow within the app itself, switching between any other tab and my Fastmail tab is laggy as well. So far as I can tell, it doesn't require usage to reproduce once the issue manifests, but it may require usage to reach that point - the main issue is that there doesn't appear to be a simple way to fix it once it does occur.
I created a clean profile to try to repro, and after logging in to Fastmail with that profile, things appear to be working normally. Presumably, after enough usage the issue will show up again, but after clicking around a bunch for a minute or two, I didn't notice any significant changes.
Obviously, having to recreate my usual profile to fix this is...less than ideal - any thoughts on why this seems to be "sticky" across restarts (i.e. is there some kind of data accumulated that isn't cleared across restarts that can be cleared manually?).
I'll keep the tab in the new profile running for a bit to see if anything changes in the meantime.