Specifically, opacity of non-images in Firefox.

http://lyokoscan.net/testcase.html is the page I'm using to test this (but this happens with real pages as well). It's just a basic loop that fades the two elements in and out.

The camera (an image) fades fine in all cases.

But the div ("This is test1") makes Xorg eat up 90% cpu. This doesn't happen on any other platform, and if I NX localhost and it goes through the NX server, I get a window that looks a lot like my normal Firefox window. The opacity test is sluggish because of NX, but it runs at a reasonable speed that is much faster than when I run Firefox directly.

I'm using Firefox 2.0.0.3, but the problem persists if I use 1.0 or anything else.

Interesting.. But is there a question, or is this just an observation?

FWIW, (neat demo btw) I can run both tests simultaneously and X takes just under 50% CPU according to TOP. The camera by itself works fine, and takes about X=6% CPU, and the red square by itself works equally fine and takes about X=20%. Quite a jump when running them both together. Hmm..
I'm using FF 1.5.0.11, nVidia GeForce4 MX440 w/9631 driver, 1600x1200 x two displays @ 16bpp color, KDE 3.5.4, and Xorg 6.9 on a 2.4 Ghz P4 running Slackware.

Well the question would be how to configure X to avoid such horrible performance

To list the same info as you have, Radeon 9550 with the ati driver, 1680x1050 + 1280x1024, I have direct rendering and GL works fineish, 24 bit color, Xorg 7.2 on 2.26Ghz P4 running Debian.

Well, you might try it at 16bpp color -- it isn't like you can actually notice any difference 99.9 % of the time.
Even though GLXgears isn't useful for proving much, it shows relativity well: at 24bpp color and my current setup, I would get around 1400 fps. Then I changed to 16bpp to increase desktop speed in general, and saw an increase to about 2500 FPS. If you don't absolutely need 24bpp color, then try this

SOLVED.

I made a bunch of minor performance improvements, like enabling mtrrs, disabling composite, etc., but I finally found the magic switch.

RenderAccel. I hadn't set it. Set it to on and the testcase worked like butter.