This is a temporary fix until ATI can sort there drivers. For most of you having issues with your FPS with ati cards the issue is the shadows. The options allowed ingame do not let you turn off shadows. ATI is notorious for having shadow issues. So the only work around is to disable them from console.

In order to turn on ingame tilde console goto your steam library and find brink. Right click and choose properties. The on the General tab select SET LAUNCH OPTIONS and add this.

+com_allowconsole 1

Start up brink and open the console with tilde key, left of number 1 key on the top of your keyboard.

Type this
r_shadows 0

Enjoy

From this http://forums.steampowered.com/forums/showthread.php?t=1882111

No, this is not a temporary fix. I have a 5870 video card and this did little-to-nothing for me. Even with the lowest possible settings and shadows on or off, my performance is choppy and not at all buttery smooth. Sometimes Fraps reports frame rates of 50-60+ yet the game feels as though it's running at around 25-30 or less. I'm just frustrated at this point, and the only thing I know to do is wait.

Well it only gave me a few more fps. Still not playable by any means.

Time to harass ati!

IS there a console command you can use to override the resolution? I run at 2048x1152 and it's not available in the graphics options.

This is not an acceptable fix, I truly hope that they are fixing this right now, as we speak. I should be able to run this without a hitch on full, what's going on developers?

Of course this isn't an acceptable fix, thats why it says temporary fix at the top of the post. From what I understand they know of the issue and are working on something to fix it.