Have you created your own client-ID for Gdrive?
Or have you used the default for now?
I'd also like to understand better your location. Eastern europe or western? Some eastern europe locations I know from feedback have sometimes had issues with bad routing (ie. they can achieve great speeds if routed via a VPN, but connection is high-latency directly).
China and some other Asian locations seems even worse in this aspect.
And as I said - the web-interface is a completely different system, so the routing to it can be completely different. This is not a diagnosis I'd want to jump to unless we have eliminated all other reasonable causes because it is hard to solve properly yourself (outside of routing traffic via VPN).
On a 100Mbit upload like it looks like you have, your theoretical max (gross) is 12,5MB/sec, so a bit under this is what you could expect to get - but yes, you are obviously not getting this in rclone right now. I have no problem saturating by full connection (about 33MB/sec) from Norway just to give you an idea, and I've seen Gdrive handle a full gagabit many times before under ideal circumstances.