Grimrock

You seems to run the open source radeon driver. It doesn't include S3TC by default. However some packages can add it (like this one http://www.ubuntuupdates.org/package/xorg-edgers/precise/main/base/libtxc-dxtn) After you'll need to set the right environment variable (for example set it in a termina...

I have no such slowdown with intel hd4000 graphics and everything at high quality in the settings.
Even if there is a lot of torchs, the game doesn't slow down at a point it lags.

It may be a bug in the nvidia driver.

How do you show the fps?

I have an hybrid system: intel hd4000 and amd 7730m, and I don't have this issue with both cards.

I use the Steam version, Kubuntu 13.04 and catalyst 13.6 beta

My only issue is that the game crash if I leave the game (alt+tab) when in a menu (no issue if I'm not in a menu).