I thought Thunderbird was getting increasingly shitty and slower/clunky, until I realised it was actually my ISP's mail server getting increasingly shit. This became immediately obvious the day that emails started taking 12-18 hours to land in my inbox. Reallllll handy for those time limited account reset emails. Funnily enough, they were planning real soon to outsource their email to another company for the low, low cost of just a few extra dollars a month, opt in now!
Transferred my IMAP inbox to my own domain, everything is now awesome again.