Anecdotally, on my high-end desktop, with 2 Google accounts and a NameCheap private email account (all IMAP), Thunderbird is sitting at 0-0.2% CPU, 2 entries in task manager totaling 190.3 MB.
It does have a quirk with IMAP, in that it only checks your main folder until you visit a sub-folder. Then it'll check those, but even then, any time I go in and click a folder, it tends to react by "really checking."
But quirks aside, performance doesn't seem like an issue for me.
A mail client taking up to 0.2% CPU doing nothing (I assume you're not interacting with it, nor is it trying to download/upload mail or similar), on a "high-end desktop", is itself a little surprising.
I'm watching it now, and it's sitting at 0%. Considering it does poll for emails from time to time (maybe 1x minute) I don't thinks it bad that it ramps up to 0.1% to do so.
Discord, Teams, Firefox, and Slack all seem to use 0-0.1% without much interaction (though I am typing into Firefox right now!) - Thunderbird seems to be 0% more often than not.
> It does have a quirk with IMAP, in that it only checks your main folder until you visit a sub-folder. Then it'll check those, but even then, any time I go in and click a folder, it tends to react by "really checking."
You can enable checking all folders by setting "mail.server.default.check_all_folders_for_new" config option. Quite weird it's not on by default, though.
Besides the sibling comment by Gare to change the default, you can do this on a per-folder basis by right-clicking, Properties... and checking the box:
"When getting new messages for this account, always check this folder"
I've used it on Linux since 2005 at least, and my only problems were importing a big Outlook .pst and having trouble with its formatting and attachments. It's open daily on my machine, and while it has become more memory hungry, the CPU usage is minimum.
On my Ryzen 7 4800H (8 core) 16GB laptop running Linux Mint, Thunderbird Mail is using 290MB-360MB of RAM. Mostly no registered CPU usage, but when I clicked around a few folders, it went up to about 3% very briefly.
It does have a quirk with IMAP, in that it only checks your main folder until you visit a sub-folder. Then it'll check those, but even then, any time I go in and click a folder, it tends to react by "really checking."
But quirks aside, performance doesn't seem like an issue for me.