I've recently switched to MacOS's built in mail client with IMAP to Gmail, never have to wait for my UI to do something. So count me in as surprised how far gmail has gone downhill.
That's the thing that gets me. So many optimisations have gone into user interface software over the years. And some of the stories of early Apple work, like 'round rects [0]' are truly inspirational.
I wrote software using Cocoa about a decade ago (so I may be out of touch), and it was clear how much thought and effort had gone into making the user interface responsive. And it generally shows.
The idea that you would just give up on that precedence is baffling. And let's face it, email's important but it's not rocket science.