I too have enjoyed Thunderbird for a long time. It's lightweight and fast which is a big plus for me. It makes showing people how to encrypt their emails very easy. The only thing missing and there is probably an addon for this is a sync with Fastmail's aliases and sending identities to give it the same composing pull-down as their web interface. I sometimes forget to manually set the "From:" to the correct alias.
I've been using Thunderbird since 2009 and while I really like it, I wouldn't call it lightweight and fast: it's pretty slow to start, takes quite a lot of memory and the search feature is really slow.
It's an excellent piece of software no matter what and I'm not sure you can do a much faster and lightweight email client anyway, because it basically needs to be a web browser to render HTML emails properly (but I'm pretty sure the search feature could be an order of magnitude faster)
Could I make a recommendation (feel free to ignore it).
There is a way to delete your search dB (its an sqlite file in your profile (but google first for instructions), and leave it rebuild (depending in your mailbox size it can take a day or two).
I had to do it once when I moved a bunch of folders.
The search should be instant.
It does take a few secs to start but a while back I created a new profile and imported my old rules etc and it's much quicker to start now.
After dropping all caches on my system, Thunderbird 102 starts in 3 seconds flat. Yes it's not lightning fast, but it's loading my office e-mails which contains 15 years of e-mails at this point.
Search is generally instantaneous on my system, including my work mails. Maybe you need to repair your index or something?
On the RAM department it's using ~750MB of memory, which is not low in any means, but considering the mail volume it's handling, it's acceptable for me.
How is 3 seconds slow? Gmail web client takes more than three seconds these days.
Thunderbird performance has never been an issue for me, except for one issue where it occasionally starts chewing CPU when syncing my monster inbox. This issue to be fair has become very rare in the last two years.
