See my last point above. Pester them. 
Contrary to what Google customer service told me today, the problem is not fixed.
What I find strange, and worrying, is that I've just tested and Firefox downloads at acceptable speeds whereas rclone mount is stuck at 1.2Mbps, after an initial burst that lasts about ten seconds.
Is Google throttling rclone? I have tried setting rclone's user agent to my current Firefox installation's one, nothing changed.
Edit: tested web interface through Chromium. Fast download speeds (hundreds of Mbps) whereas, with the same file, an instant before, rclone was crawling at 10Mbps...