heres another optimisation for ya.
Got it off the planetdoom web site.
Find your 'DoomConfig.cfg' file and open with notepad (make a backup as you never no)
find line:
seta image_cacheMegs "32"
try changing the 32 to something higher (within the amount of ram you have) so for example put
seta image_cacheMegs "96"
should get a boost.
