I've been running Thunderbird in the background without problems the last 15-odd years on whatever computer I had at the time. Still do. No performance problems running 20+ mail accounts with loads and loads of mail. Ofcourse it can be slow if you do things that requires TB to recreate the mail storage but other than that I have no problem opening mail and reading it fast. I'm using both IMAP and POP3 accounts mixed.
I believe there are settings you can run that might create more problems with performance but I haven't touched anything the last 5 years so can't say what it was any more.
I do not think I have touched anything either since I like keeping as much at default as possible, but maybe I have and just do not know it, but it is dog slow for me.
I believe there are settings you can run that might create more problems with performance but I haven't touched anything the last 5 years so can't say what it was any more.