MacOS – Limit the upload speed at OS level

This is a limit imposed by Apple.

Cellular downloads are limited to a file size of 20 MB or smaller.

See comment below, iOS 5.1 has increased this limit to 50 MB.

Is it 600KBps (600 kilobyte per second) or 600Kbps (600 kilobit per second)? For the latter it would take 5 days to upload 30GB with optimum upload speed. You say it took 10 days. I would say that is reasonable. You can use the System Monitor app to monitor network traffic. It doesn't make clear what app is using the network, but it does make clear if upload is only half of what you would expect.