Having the same issue. Can't get my FPS above 15 in-game regardless of my settings. Have tried fiddling with the effects of Unity 3D, running in 2D, without Compiz and so on. Nothing effects the FPS even the slightest.
EDIT: Forgot to mention I've repeatedly re-installed my Nvidia drivers to make sure it's not an issue on that end, trying different approaches in the process. Get a smooth 550 FPS in glxgears through the nvidia drivers using optirun.