http://www.virginmobileusa.com/mobile-broadband/
It says all over the place on this website that they will throttle you if you use an excessive amount of internet.
" As long as you're receiving coverage, average download speeds will be between 600-1400 Kbps, peaking at 3.1 Mbps and average upload speeds will be between 350-500 Kbps, peaking at 1.8 Mbps.
If you have reached the established threshold for your plan, your speeds will be reduced to 256 kbps or below for the remainder of your monthly cycle. Speeds will be restored once a new plan is purchased. "