I can think of a number of causes...
1. gtk-qt-engine. It's nice, but it's also slow. I would recommend using a different GTK engine.
2. Dynamic CPU frequency adjustment. On some chips, the p4_clockmod module has to be used, and to put things bluntly, p4_clockmod sucks - it slows down your PC and doesn't give you much in the way of power savings. If your CPU needs p4_clockmod, I would disable frequency scaling altogether.
3. Firefox using Pango, which makes it slow. To remedy this, put
in your .bashrc or .bash_profile.
4. EXA. The new EXA hardware acceleration method doesn't use the best defaults on all architectures. If, for instance, you have Intel 9xx graphics hardware, you should put
Option "MigrationHeuristic" "greedy"
In the "Device" section of your xorg.conf. This will speed up scrolling significantly on affected systems.
5. Swappiness. Default for the kernel is 60 (range is from 0 to 100). If you have a lot of RAM, this means it may be being underutilized. To set it to a lower value, put
vm.swappiness = X
in /etc/sysctl.conf, where X is some number like 5.
6. Firefox... being Firefox. The sad truth of the matter is that Firefox on Linux is slower than on Windows, probably because most development is focused on Windows. Also, I should mention that slow scrolling on GMail and Facebook is a known bug in Firefox 3 on Linux - if you use Firefox 2 (some distros still use it, e.g. PCLinuxOS) you'll notice a decided lack of sluggish scrolling.
If you're not averse to using a proprietary browser, you could try Opera; it is *much* more responsive than Firefox, and uses KDE's native graphics toolkit to boot. If you want to stick with FOSS, Kazehakase might be worth a look - it is quite ugly, but it gets the job done. Unfortunately it's not in the repositories.
(There's also Arora - it is in the repositories, but can't use Java or Flashplugin as of now, although it's blazingly fast. And, last but not least, there's Konqueror - the KDE file manager is also a web browser, and a pretty nice one too. Unfortunately, Flash is unstable on it, and some pages, e.g. Facebook, aren't handled entirely right.)
7. Likewise for Flash. Flashplugin for Linux is kind of crap. Unfortunately, much of the internet's content is now in Flash, due to what stupidity I know not.
That's all I really know of - there are other tweaks, but Pardus, being a good distro, already employs them.