Is CFS in Linux broken, or is Xorg broken?

[Fred .]

I am running Ubuntu 8.10 with kernel 2.6.27, Xorg 7.4, Firefox 3.0.4.
I have a Intel Core 2 Duo 2.13 GHz system (dual-core, 4gb ram)
I clicked on a SVG image, and Firefox froze for several minutes. (
http://en.wikipedia.org/wiki/Image:Unix_history-simple.svg )

GNOME System Monitor said that core #1 used 100% CPU, and that core #2
used 0-5%.
But my whole desktop was slow and sluggish and a pain, it was
crawling, it was very slow.

Something is wrong here. Why was my desktop was so slow and
unresponsive, when I had a whole core free under no load?

Either the scheduler in the Linux kernel must be doing something
wrong, or the Xorg must be doing something wrong.
Which is the culprit? Where do we put the blame?
Either way, this needs to get fixed. Desktop Linux cant be this way.