I've been a ProtonMail, and Beta ProtonMail user, for some time now. I do feel the Beta brought a lot of improvements, but for some reason they removed the VIM bindings halfway through the Beta and never reintroduced them. It's something that seems small, but really interrupts your flow when you can't use the j/k keys to scroll through your email on the web.
I highly recommend Tresorit. I've been using it for just over a year and couldn't be happier. Simple interface and the app seems very performant (macOS). At least I haven't noticed it hogging resources.