This issue seems like it has been going on a while. I will try and explain in dummy terms. 
I use FireFox for my browser. Lately when I go to a web site, the page loads quick but I am not able to scroll for a while. I noticed there is activity going on by the activity lights on my computer. It seems like I can’t scroll until this activity slows. I was curious to what was causing this activity so I brought up task manager>processes. It was Comodo Internet Security using 50-70 % CPU. Every time I switch web sites this goes on. I know there has been some program updates lately, but this didn’t start until recently. I have A/V scheduled scan turned off, instead I do scans manually. I do have my data base update on auto. I guess my question is Comodo A/V doing scans everytime I go to a web site and then go to another. I have Vista, my Fire Fox version is 4.1. The reason the version number is I use FF primarily and this seems when this issure occurs.
Have you tried creating a new Firefox profile or starting firefox in Safe Mode to see if the problem persists? Also, which extensions/plugins, if any are you using?