Unfortunately, I've come to the opinion that Proton Drive is not ready for prime time.

I marked about 100GB of files for upload to my drive (I pay for the premium service). These are important files that represent years of work, not just rando MP3s or shit like that. It's been days and they're nowhere near uploaded. This was after I gave up on using files I had uploaded previously to do a restore, because it took over a day to download them and half the downloads failed to complete. I've come to the conclusion that Proton is really only useful for archiving or storing a relatively small amount of small-file documents, it's useless as a cloud backup. I'll have to revert to Google drive for that, I guess. Overall, disappointing. And this is on top of the issues with email taking hours to populate. I fully support what Proton is trying to do, but this level of performance doesn't qualify as professional.

UPDATE: All the folders I selected appear to have synced, but I haven't tried downloading anything.