Multicore Rendering has nothing to do with it, neither did my CPU. I upgraded to an i5-3570k for Christmas. I'm not sure what else to try at this point.
I'm having a similar problem. Moving the mouse results in the character turning about half a second later. Also on Arch Linux with ATI, running XFCE w/Compix+Emerald as my WM. Video is smooth, but input is super laggy.
Issue transferred to ValveSoftware/Source-1-Games#43.
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More recently I'm finding that my mouse and keyboard lag. It's at a point where it becomes unplayable. Though, it does happen occasionally. At first I thought it was because I was running the game Windowed but full screen. Turns out that didn't matter at all after I switched it to Full Screen in game and experienced the same issues.
A bit more detail on what happens. If I were to move my mouse around in circles, the mouse would stop and pick up a few seconds later. The keyboard would do the same, but act as though it were tapped in the last direction I tapped after I get control back. There are times where it will act as though I'm still holding down Mouse1 at times as well.
I'm running Kernel 3.6.10-1 with the 12.11-beta11 ATI drivers. I've got a Q6600 and a 5870. AwesomeWM is my Window Manager and I'm using Arch. My mouse, if it needs mentioning, is a Razer Lachesis and this issue doesn't happen outside of the game [I've tested while the game is running]. While it's happening, I maintain over 100fps. Would Multi-Core Rendering have anything to do with it?
Any other information I need?