The first and most obvious point is to be sure your seedbox isn't just limited in upload - but I presume that is not the case? .. Google Drive is certainly capable of far more.
You already use a good chunk size and your own API, so that should cover the most important performance options for uploading.
Have you tried uploading with the same configuration but from a different PC (like your home system) ? Are the results about the same then, or very different?
Otherwise I would suggest adding -vv at the end so we can see debug-info. You can also use --log-file=MySpeedLog.txt to dump it to a file. Try to keep the log short - maybe a minute of transferring or so. Debug logs can get pretty big and long, and any problems under the hood should become apparently even in a short-duration test since this is a repeatable problem.
If you have full control of it, then just speedtest.net (assuming it has a browser).
If not, then presumably you have some other way of transferring files to you right? Maybe it comes with an FTP or something you can just try downloading a large file from and looking at what speed you get?
This may also be an option if you only have the terminal to work with:
I don't have a seedbox myself so I'm not sure how they typically come set up. I imagine it's a fairly barebones Linux VM? Basically any other alternative that we can use to pull a file from the system would work - as long as you can monitor the transfer speed.
I would also imagine that it should be fairly easy to check with your provider what your seedbox should be capable of. I would be very strange if someone sold a seedbox that didn't have upload/download capacity listed first and foremost in it's specs.
In any case, be sure to not confuse Megabits (Mb/sec or Mbps) and MegaBytes (MB/sec or MBps). I assume you probably know the difference, but you said "mb/s" which makes it a little ambiguous what you mean. Sorry, I'm not trying to be nitpicky just for the sake of it
