I have to use Thunderbird for work and I wish it was 1% as good as Gmail. The search sucks, it's slow to receive mail, it crashes quite often. Either your Gmail is profoundly broken or you have a magic Thunderbird. In case the latter is true: do you have any tips to optimise Thunderbird?

Have been running Thunderbird for G-Apps based work email for over a year (Fedora 26,27). Using the Provider for Google Calendar extension works well for integrating calendar into Thunderbird.

Other than fighting with the calendar occasionally (sometimes needing to force a manual sync to see coworkers' events), Gmail in Thunderbird has been pretty smooth. I have not noticed slow search, slow mail receipt, or application crashes. In fact, the search is often too good, in that after running it it matches hundreds of more emails than I usually expect. I usually stick with the quick filter which is a bit less flexible, but generally returns more useful results.

Another advantage, on Linux at least, is that if you copy your ~/.thunderbird folder to another machine, all accounts, GUI layouts, settings, search results, tabs, etc move over flawlessly. I think you just have to re-sign in for Google accounts on the new machine and you are good to go.

I would check that Google isn't ratelimiting you, and I would also check that your hard drive/SSD and RAM aren't dying. I've experienced issues like what you describe, but the first time was caused by a dying hard disk, and the 2nd by Google limiting the number of IMAP connections to a hilariously low number.

Thunderbird is quite a piece of crap. Apart from the core, everything else is written in javascript and thus it is a single-core web application.

I have decided to just leave it be, and let it hog my computer, because it interface decently with google calendar.

That being said, I can definitely vouch for clients like Claws Mail (a bit ugly, but does its job) and Evolution (super fast, but it's written in C#/Mono)