Last time I installed Thunderbird seems to have been in the 00's sometime[0]. Since then I have used the same install, only updating it, and moving/cloning to other units.
Back then PCs were s-l-o-w but Thunderbird was _fast_. Moreso than Outlook/Outlook Express in startup as well as operation. Then, speed deteriorated bit by bit over the years. Due to feature creep, I suppose (in other news; also UX in several subtle ways that all add up).
I do not believe that the claim of "Fastest ... ever" is credible, and I have reason to believe that speed on an 13yo X86 unit still running 32bit[1] will not be impressive. Still, if speed is improved even slightly I'd welcome that.
[0] Datestamp of earliest email in a random folder is December of 1999 - 23 years ago. But then the earliest emails were imported from another email client into Thunderbird. In any case I started using Thunderbird as a very early adopter, mostly due to their very convincing Firebird product (I was in Tech then, too).
[1] The unit I prefer using for anything that does not require Windows or other ressource hogs (eg. browsing like now). Yes it's _that_ old. It's Linux, so no problem.