I've found that keeping all my old mail--including from old dead accounts--in Thunderbird has been a big part of the performance problems I experienced. Having been using Thunderbird from the very earliest of days (after migrating from SeaMonkey), I had a lot of dreck from old POP accounts and newer IMAP accounts.

I've switched to having it do full downloads of messages from IMAP, and run MailBackupX separately to ingest everything. Every month or so, I delete everything older than a month from Thunderbird, and rely on MailBackupX for my historical mail reference, searches, etc.

Now Thunderbird is fast and responsive.

My partner keeps hundreds of thousands of emails in Thunderbird from tens of email accounts, some active, some dead. Last time I looked, it was well over a terabyte of old email. The computer, a reasonably recent and fast Mac, can take several hours to start Thunderbird. That being said, I'm impressed it can handle that much cruft at all.