Ok, so I bought it during the sale a couple of days ago and only just got around to installing it. Unfortunately, it seems the game runs unplayably slow for me, but not right away. It's after I go down the elevator at the beginning that it turns unplayable. By unplayable I mean it's probably about 2 FPS. Literally it's that bad. Ok, so at first I had the high res textures installed and I was using the widescreen tool to set it for 1920x1080, so the first thing I think is maybe this pushes things just a bit too far for this videocard as it's not the most powerful in the world (HD5570) but really, it should be more than enough for this game with plenty of room to spare, so that wouldn't really make sense. Still, just in case something there was causing the problem, I went ahead and removed them, running the game at a standard 1280x1024 with a standard FOV and the original textures. Still slow. So I give up and uninstall the game, then reinstall it from scratch such that there should be no original files left in case anything was modified. Same thing.

I googled around a bit and I did see mention that it doesn't like SMP systems and people are suggesting changing processor affinity to just one core. I was about to test that the simple way -- changing it in the task manager -- when I notice it's already set to just use one core in the task manager. I'm guessing GoG already integrated that fix. Now, I don't have the most amazing CPU in the world -- an AMD A4-5300 APU (3.4GHz base frequency, but the APUs seem to be a little worse per tick versus many others, so it's probably more equivalent to something like 2.8 or so) -- but it should be sufficient to basically just blow this game away. I originally ran it on a CPU that was single core and I think at most it was the 2.4GHz Athlon XP CPU I had for a good while (more likely it was long before that and was the 1.6GHz or maybe it was 1.8GHz Athlon that I had before that. I can't really remember exactly as it has been a long time.) It's worth noting that I actually had an old, much modified copy of the game around before my latest upgrade to that APU and it ran fine on the same videocard paired with a Sempron 145 (2.8GHz single core) albeit I hadn't tried the high resolution textures at the time (I forgot about them.) I can also say that Thief 3 actually runs with the high res textures fine and it uses much the same engine, so this makes me think that this videocard and CPU combo is more than sufficient for DX: IW even with all the enhancements.

Is there anything I'm missing here that could be causing the particular issue? BTW, my power settings are kind of custom, but basically closer to the performance mode than anything else (basically just allowing things to scale down when not in use, but allowing them to go all the way up to 100% as needed.) Every other game I've tried that reasonably could run well on this system has done so.

EDIT: I did notice one thing though. The interface feels a bit sluggish throughout most of the menus even before I ever get down the elevator, so whatever it is still is having a bit of an effect on the whole game I guess.