i did it on a wiped memory card, so no special items, or unlimited ammo, if you wanna do a speedrun like that, the only real thing you need to know is, keep running all the time, don't stop and fight zombies, you can pretty much run around every zombie, and apparently running without a weapon in your hand speeds you up a little bit..i did that but im not quite sure whether it helps.
another thing is, aim for the worst ending, that way you skip the final battle with the tyrant and just get helicoptered away.
if you go for the worst ending the game itself is not long at all.
ofc i have completed the best endings aswell.
also i know the game back to front, so i don't ever stop to think, "where do i go next"..or things like that.
don't bother with the pistol ever, the only weapon you need is grenade launcher, and thats only for the bits when you actually have no other choice but to stop and fight.
the underground battle with the spider can be avoided when you have to use the knife to cut away at the web, just get him into a corner, then run to the door, hack away, and if your quick enough you will get through first try.
i cant think of anything else atm, but i will get back to you with more bits that can be avoided..
where abouts are you atm anyways?
oh don't try and get any of the MO disks in the lab, that whole section takaes away alot of time, you wont be able to get chris out, but you save alot of time, and you still get the rocket launcher.