That's what I've done. I disabled the fader effect. But even with the fader effect disabled, here's my system monitor.
I opened firefox with google, took 1st screenshot, then changed url to my website. Then I tried ubbedall template on his website, and dark matter template on mine.
With google website in firefox:
with my website on raia template:
With http://www.ubbedall.dk/Raia/
My website with Dark Matter template
I found that the problem seems to be the way firefox render webpages. If firefox window is minimized, cpu drops to normal. And when firefox is maximized on screen, then cpu is 50-60%. The fact that the template use some div on top of each other, and shadow seems to make firefox go crazy on cpu. It doesnt seem to affect other browser, so let's hope it'll be fixed in a future firefox release