I'm on Win10 x64 using
--bwlimit 4M with my upload. However the result only got under 3MBytes/s, which is not ideal. Full command can be found on the title bar of the pic.
According to taskmgr, rclone was using on-off mode to control the rate.
40Mbps is my ISP's upload bandwidth limit, so it's not about insufficient bandwidth but the way rclone managed to limit the speed.
Maybe switching to other control method will make it more accurate on result.