Or, test your bandwidth with, and without a firewall. I think you'll find the difference to be negligible, unless your system resources are seriously deficient. Turn a firewall back on ASAP!
To test bandwidth (internet connection speed), I use the one at CNET (USA) but you should use one that's closest to your location.
http://reviews.cnet.com/7004-7254_7-0.html <Bandwidth Tester>