A recent (relatively speaking) redesign of Gmail.com left it less usable and much slower than before, and that drove me back to Thunderbird. Even on a high end machine with fiber, it takes a while to load for me. I still go there occasionally to use the superior search, though.

I think part of this is just time to get used to something new? You might notice some improvements too, for instance only after I started using Fastmail I realized the Gmail UI can be pretty slow and heavyweight although I never noticed this before.

mutt and thunderbird are the only 2 clients I have ever found that can make mailing lists usable - gmail threaded view sucks for conversations that have branches, and encourage terrible quoting / top posting practices.

vivaldi, the successor to opera, has a client in beta. it's not as good as the old m2 client in my opinion but it's quite usable. my main gripes: keyboard navigation still isn't on par, the default is to top quote, no highlight to quote, and no encryption support. this latter doesn't bug me much because no one I know uses it. but it's fast with good search capabilities, automatic indexing, contacts, etc.

Which of course you improve upon need /I(rony)

Edit: after I wrote the above line, I realized that many people use closed source software... Well ok then.