I just tested rn with DropBox, GoogleDrive, and OneDrive, all with their native desktop apps. I simply put a 300MB file in the folder and let it sync.
DB: 500 KiB/s
GD: 3 MiB/s
OD: 11 MiB/s (my max bandwidth with 100Mbps)
I don't know what causes the disparity here, but I have been annoyed by this for years, and it's the same across multiple computers I use at different locations.Another funny thing is if you just use the webpage, both GD and DB can reach 100Mbps easily.
Edit: should mention Google's DriveFS can reach max speed too, but it's not available for my personal account (which uses the "Backup and sync for Google" app).