I had the same problem with a core2 duo E6600 and an ati 4850:
No matter the setting, framerate was generally below 20 and even dropped below 10 at some times, settings at auto detect (which was low for most settings).
I fixed this by upgrading to an I5 3470. Now not only does my framerate never drop below 20, I’m also able to run most settings at high, with the same graphics card!
They seriously need to fix their cpu bounds and/or increase the minimum specs to something like an i5.