Actually, Resilio is not any slower than Dropbox. Syncthing is also faster thans any commercial offering I ever tried. Even Nextcloud sync is fast after an initial init.
What exactly are you referring to? Upload speed? That's not the issue.
Dropbox is the best at crawling a very complex directory structure with millions of files and diffing it efficiently with the server and other clients, and then transferring all the individual changes. Nothing else comes close. And their write-ups show the work involved:
