It's because Brink's engine hasn't been optimized, especially for the greater populace outside of their own company where they probably perfected to the max for that internal testing, once it hit the streets, they had a lot on their hands to fix and optimize.
If you really are getting only 15 fps, drop the console with "alt+ctrl+~" and type "seta r_shadows 0", if you further need to, though I doubt it disable motion blur, ambient occlusion, and post processing, post processing if it hasn't been fixed yet, make sure to set that to off right after you launch it, otherwise, after you play your first game or experience any game-play during that launch you'll need to restart to change that.