I recently received the following comment and I thought it would be a good idea to go over some reasons why a web browser might get sluggish.
WTF, did you test this page? It is so freakin slow to scroll in FF3 – your layout sucks 100%, seriously.
The most common reason is that you have may have too many tabs open. Sometimes it could be a buggy extension you installed; While most extensions are well done, there are a few that are really not ready to be used. One of your tabs that has a lot of JavaScript or Flash going on. Sometimes if you have a Quicktime plugin loaded in another tab that can bring your browser to a crawl.
There have been times that I’ve left Google Reader open all day at work in a tab and sometimes it will just end up making my browser so slow I have to relaunch it. Although it hasn’t happened that often with Firefox 3.
I would attribute the slowness to your static fullscreen background image. It’s not very well known, but a fact nonetheless, that using these static backgrounds causes dreadfully slow scrolling on many browsers. I suspect your reader had a legitimate complaint.
I doubt the number of tabs had anything to do with it, this is reproducible with only 1 tab open.
I think the transparency might compound that issue as well. But I’m also guessing their videocard (onboard or not) may not have much memory, since it doesn’t seem to be a issue for me.