Pcloud is one of the services that support block-level file copying. This is also fairly clear from the synchronisation times of the official client. For example, updating the tags in MP3 files does not transfer the whole file.
Obviously, having this in rclone would significantly reduce the transfer times and bandwidth usage for file updates.
I was going to create a feature request but decided to first get some feedback here on the forum.
Do any providers in rclone support this? How difficult is it to implement?
Before even thinking about it do you know if Pcloud API supports it? One thing is that their sync client can do it. Another one is if their public API allow 3rd parties programs like rclone use it.