Thunderbird has not been, in my experience, slow. It is also configurable "out the wazoo" so you can eliminate seeing the vast majority of the defaults you don't want to see if you look up how to turn them off. When I've been installing Thunderbird for my blind and VI clients I do not set up the calendar feature, Lightning, unless they say they actually use an electronic calendar.
Also, if at all possible, when setting up e-mail accounts these days, one should always use IMAP access. This eliminates the need to import messages entirely when a new e-mail client is set up, since that happens all on its own after the client syncs with the server.
It's also worth considering using your e-mail provider's webmail interface, too.
Brian - Windows 10 Home, 64-Bit, Version 1709, Build 16299 (dot level on request - it changes too often to keep in signature)
The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr