I honestly don’t load it that often - it just sits there in a open tab all day. Viewing, moving, composing messages (esp with keyboard shortcuts) is literally instant for me. I have never noticed it being anything other than fast. Same for their iOS app (which I think is largely a web view).

How many things are you moving for it to take that long?!

I accidentally marked everything in a folder as unread recently and that did take a while, but I did get a very helpful progress bar while it chugged away.