Put the tweak.cfg file in your doom3\base directory.
EXECUTE it in the game while pressing [p]
if that don't work no fear sobig is here, open console with [ctrl]-[alt]-[_] and type "exec tweak.cfg"
notice that you can change all binds!,
the binds without information are recommended so don't change them if you want a performance boost.
if you want to autoexecute it, copy the lines in tweak.cfg and paste it in doomconfig.cfg
notice that you first delete the commands that are allready there.
Doom 3: Extreme Performance Boost (English)
Описание и инструкции