ALL YOU HAVE TO DO to speed up your Doom III is to change one key in Doomconfig.cfg file and your game will go much faster, it's claimed.
We haven't had a chance to try this out ourselves, so do so at your own risk.
The report is that if you find Doomconfig.cfg file in x:\Program Files\Doom 3\base and open it in Notepad, and edit one line, the game will go faster.
When you find seta image_cacheMegs "32" you change 32 number to 96 or even 128 or 256 and your Doom will go faster, it's claimed.
These guys
here that found this key haven't tried any benchmarks yet, so it remains to be tested.
Source:
The INQ!
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