There are many reasons why your computer can be sluggish. The quota will only affect Internet activity, not your system's speed in general. The last reason to suspect would be that you've exceeded the bandwidth limit. Viruses, spyware, adware, or just too many programs running at the same time can slow down your computer.

To check if your computer's bandwidth is limited, use the web tool at:

http://www.columbia.edu/cgi-bin/acis/networks/quota/netquota.pl