7:59 till the credits roll. I can see speedrunners try and go out of hiding early to get the slug to turn back and follow them for a couple meters before it loses line of sight and returns to its patrol to get a couple extra seconds free of slug monster. ;-)
This was one of the games where I pulled out notepad.exe and wrote down the order in which I would collect or use items. I suppose this is the way it was meant to be played, almost like a rouge-lite where instead of respawning with better gear, you respawn with better map knowledge. The first actions and items were easy to find. I only had a hard time finding the gas canister. To avoid having to search for items in the late game under pressure, I tried to identify all the items in all rooms early on (where they lack highlights) and it seemed the front pump room only contained an oxygen candle at first sight. (In fact I found the secret note before the jerrican. :p) This way I also missed the exhaust pipe, but it quickly dawned on me, that the Pipe Dream may not have been as empty as it seemed at first.
Overall I enjoyed the amount of playtime I got out of it thanks to having to figure out a sequence of actions that keeps oxygen and tilt in check. There may have also been a depth limit in place, but such a time limit would be more of a psychological factor, since you usually run out of O2 or don't reach the pump in time instead of getting depth crushed.
Speaking of pumps, there was a confusing effect where when the front was already lower and I tried to used the front pump, the tilt would be worsen. Other than that I encountered no issues!
P.S.: On my first attempt I took so much time with every room, afraid of reaching that point where the monster goes active, that the sub tilted before I even got to see the slug. ^^