On the map known as Steamland, I have always experienced severe general lag. It feels like it runs at about 10 to 20 frames per second. I just updated to the AMD experimental beta drivers on Ubuntu, but this hasn't helped. Some maps are better than others, and most are generally fairly snappy (30 to 60 frames per second).
I also get a severe frame rate drop when Husks shoot their fire guns. It doesn't matter which map I'm on. I guess it's the introduction of fire particles.
Other than these two instances, the game runs quite well. I still wouldn't mind seeing a general boost in frame rate, though.
Regardless, thank you, AMD Linux team, for your hard work in improving drivers.
Ubuntu 12.04.4, 64-bit
Unity Desktop Environment (vanilla)
Asus M4A78LT-M LE Motherboard
Asus BIOS Version 0803
AMD Athlon II X4 645, 3.10 GHz
AMD Radeon HD 6790
Catalyst Control Center Version 2.20
ATI/AMD Proprietary FGLRX Graphics Driver (**Experimental** Beta)