Hi guys,
I got this game this morning after many months of anticipation. I found the game to be extremely choppy in single player / multiplayer.
My specs include a Intel core 2 dual 3.33ghz, 8gig 1033mhz ram and an ATI 4870 1 Gig.
I looked around on the forums and noticed that many other users with the ATI brand of cards were experiencing the same issues.
I tried installing and rolling back various driver sets and have found 11.3 to be the best so far.
After messing around with the cvars, I came across a temp fix until the people at SD come out with a patch of some sort.
You have to enable cvars by setting the following launch options on your brink shortcut (I did mine thru steam, sorry I don’t have info on how to do this with other retail copies but the googles will help).
+com_allowconsole 1
When the game loads and you enter into the char selection screen, hit the tilda key ~ and disable post processing by typing
r_skipPostProcess 1
Simply disabling the postprocessing through the ingame menu did not work for me and I have had to do similar things in the past to get various games to function at a framerate I consider acceptable.
Additionally you can disable shadows by typing
r_shadows 0
This might increase some performance with those that do not have a powerful CPU.
My FPS jumped from around 30-40 to over 90 using this. By disabling the post processing through the console (you cannot do this in game), you are cutting out the shaders as the bottleneck when rendering the frames (which I suspect the great majority of users here is experiencing)
On a side note, when playing single player, the game uses bots which are extremely cpu intensive. The difference between a solo match and a multiplayer game is night and day as your pc does not have to calculate 15 of so bots.
See you in game 
MTJ