I needed to transfer 5 TB worth of data out of Mega to Google Drive and I initially was transferring at around 4 MiB/s (32 mbps). I discovered that I can just use the "--transfers=" and set it to 100. Although each of the 100 files faces the slow Mega transfer speed of 1 MiB/s, it didn't matter since I was working with thousands of files. Obviously, if you were transferring fewer files, or even 1 file, you would encounter slow transfers.
For example:
rclone move Mega:5TBFiles GDrive:5TBFiles -P --transfers=100