I suspected so, it seems to be something on the road from your PC to OneDrive. Could be your PC setup, local network/Wifi, ISP or OneDrive (throttling) - most likely one of the last two.
My best guess is still an issue/limitation between your ISP and OneDrive since the speed is significantly below the typical speed of OneDrive and your other clouds (e.g. GoogleDrive) are much faster.
Just double checking: Are you sure your max. upload speed is 20 MByte/s (roughly 200 mbps) and not 20 Mbit/s? An upload speed of 1.2 MByte/s is sometimes what you actually get on a (ISP promised) 20 mbps connection.
I did a performance test last year and found that OneDrive has a limit on the number of file creations per second and that 4 transfers were sufficient to saturate that rate.
My second guess it therefore that the slowdown could be some kind of "soft" throttling triggered by the combination of many small files and --transfers=10. I therefore suggest you try using the defaults for a week and see if that helps. It sometimes takes days for throttling to fully cool down.
If it doesn't help and your tests show significantly improved performance with 10 transfers, then this is of course the right setting for your situation. Do you happen to remember how much 10 was faster than 4?
I doubt it, especially if the rclone clients are the same version. Perhaps your phone is using another WiFi (band) or a cellular connection with another ISP? Perhaps it is due to variations in downloads from Google Drive as discussed in this hot thread.
It could also be pure coincidence, I do sometimes see such variations (factor 2) hour to hour and day to day.