I can't seem to get any consistency in the time it takes to get through the room of flood after the elevator drop in the beginning of the level. Sometimes the door unlocks after a few enemies, and sometimes flood continue to spawn for minutes. Is there some specific trigger?
To add to this the above, It sounds like you're talking about Lab fight, which is hard to figure out the timing for if you don't know what to look for. I'll paste an explanation of the Lab fight from Mono.
Monopoli: kill the bodies, kill the grunts, kill the heretic, kill the fatties, kill the carbine heretics, kill 4 flood, wait 1 min.
essentially, as I understand it, once you kill the four fatties in the first enemy wave, the carbine heretics spawn at the upper door (where you came in). Then once you kill them the final flood wave spawns. Once you kill 4 of those a 1 minute timer starts. You'll see the top runners will usually glance over at their computer screen at this point, They're looking at their timer so that they know at what time they should kill the remaining flood in the room. If you kill flood during this 1 minute timer period, the room will keep spawning more flood to replace those you killed until the 1 minute timer is up, at which point no more reinforcement flood will spawn when you kill the remaining flood forms.