I know this is a cold thread but, here is my 2 cents;
First I was a dropbox user (BTW I had like ~500GB data with ~800k files on average on all cloud providers).
I wasn't happy with client app resource usage and had problems with big amount of file operations (like mass deletes, mass copies etc.) even on their website.
So I moved to OneDrive for a year (it was coming with free Office 365 after all, it was a bargain with same pricing with dropbox). Then, this happened (See attached screenshot) and all I did was simply; updated the synced folder options. And UI was very very unresponsive all the time, I don't know if it was because of the amount of data I had.
Then I moved to google. Still not very happy with client resource usage but at least it was responsive, options was ok. Best among these 3 so far. I don't know the other 2 had any updates, improvements after I left them but since it is 500GB of data, I kind of am reluctant to test the other 2 again.