From my limited testing with Onedrive, i found it to be kind of similar in performance as Gdrive. Slightly higher transfers quota and good bandwidth.
What happens if you run this with completely standard settings? 12 transfers is probably far too much to be effective Onedrive, and tpslimit 3 is very restrictive. Why set it so low?
EDIT: Also - have you set up your own clientID for Onedrive? This could certainly affect performance. I have no idea what the shared common rclone quota is for Onedrive is - ie. if it's ample or not. Would have to ask Nick about that.
EDIT2: I did a quick test for you and I can replicate your issue. I am having a hard time breaking 20MB/sec no matter the amount of transfers. I can't recall noting the bandwidth was that poor, but when I think more about it I think I tested connection-limits more than bandwidth, so I might just be mistaken.
Otherwise it is possible that this is something that has just changed - or it could concievably be high load due to Corona-related thing. I have heard that Google had some minor issues in some systems (including Gdrive) from a high spike in traffic recently. I guess a lot of people suddenly had a lot of time to sit on their computers and do some cleanup ...
So I am not sure about how to "fix" this - except that you probably want to keep an eye on it and see if it varies throughout the day-cycle and in the comming days (in case it is due to high load on the systems).