Another weird shortcoming is that there is no way to collapse folder tree... which can be annoying if you have a huge folder hierarchy on your imap server.

Due to this, and due to the slowness logging in, I looked for alternatives... and found:

claws-mail

It's UI is a little bit rough, and takes some time getting accustomed to it, but it has everything that the built-in mail hasn't: fast log-in, collapsible IMAP folders, light on the CPU, etc. And the non-standard scrolling is actually more convenient (faster) than the standard way.

So for those who prefer function over appearance, here is your solution.