I've also been in the process of moving off both Dropbox and Google Backup & Sync because they are such resource hogs on the Mac. iCloud drive isn't perfect, especially when it comes to sharing, but I'll accept that if it means my fans aren't going to be on full blast when it's running.

I can't remember a time when I've had Dropbox open and not found it at the top of the CPU tab of Activity Monitor. This may be because I have a couple symlinks (not many, just some dotfiles) or because I have a large number of files (nothing ridiculous, ~100k photos backed up) but the fact is, it's been long enough for Dropbox to have figured this out and fixed it.

It's bad enough to where I now leave Dropbox off all the time, then only turn it on when I need to sync something. With iCloud sync I don't even think about it, which is the way it should be.

I opened Dropbox before starting this comment to see the new UI and even after giving it enough time to catch up, it' still using between 48 and 127% CPU. It makes even Outlook feel breezy.