I’ve the exact same problem but with the Google Drive client. Download is fast, upload via the web interface in Chrome about ~200Mbit/s, but uploading large .zip or image files via the Google Drive client maxes out at ~700Kbit/s.
Maybe someone can confirm this:
Downloading a lot of small files from Gdrive to my home PC hits 403 rate limit errors instantly (except if I set --transfers=1), doing the same from a Google cloud compute server there appears to be no limit (same rclone version, same credentials).
(Not sure if that’s the same problem you guys are getting)