I have wanted to migrate to using Proton Drive for some time, but each time I experimented with it I ran into bugs/slow speeds. However, I am happy to report that with the latest updates described in the link below that it is massively improved and nearly as fast as my experience with iCloud ADP.
For real world experience I was able to upload all my data I wanted (around 100GB) by just setting it up overnight and by the morning everything was there. This includes >15,000 photos uploaded directly from my iPhone. I have not fully moved to Proton Drive and cancelled my iCloud+ subscription.
If you have been similarly interested in migrating to Proton Drive but disappointed with experiences in the past, it may be worth giving it another look with their recent updates.