First off, some observations about player states. When a player is fading in or out of a level, they are still free to act from the moment the level loads to the moment it unloads - they are not locked in place, and can take any actions that would otherwise be available, including moving, shooting, using items and grenades, operating doors, picking up or dropping items, reloading, etc.

This leads to the first speed tech, which I've termed "Locking In". If you pause your game immediately after using a hatch, you can cut off quite a bit of time, a few seconds per hatch. The IGT timer is paused while in the pause menu, but events like fading in, fading out, loading a new level, and dying will still proceed in the background. By pausing after using a hatch, you can wait for the next level to load without running down the IGT timer.

In addition, it is also possible to skip part of the fade in. Once the graphical effect of the fade in for a level starts, i.e. once you can see it "fading in", pausing will allow this process to finish without spending any time. This is very useful when attempting to move before the level has faded in ("Fade Walking"), as it is possible to run into enemies before you gain vision, and this gets you a little extra time in that scenario to get your wits about you.

The player's dying state has similar properties, with two exceptions - you cannot fire your gun, and sometimes you cannot reload. You can still enter new levels, use grenades, etc. Entering a level while dying will just make you die once you load the new level.

Snakeworms can spawn just about anywhere, including inside walls, inside turrets, and even inside other snakeworms. In this state it is possible for them to squirm through solid objects.

All enemies can see you through walls in some circumstances. I believe this occurs if you get too close to a wall and your viewbox pokes out the other side.

It is possible to postpone a fight with an imp or crawler to a later room, even though they are faster than the player. Because both enemies are briefly forced to stand still and make their alert sound when the player enters their field of vision, by having them chase you down a route with lots of sharp turns and right angled corners, it is possible to make them do this repeatedly, allowing you to let it chase you until you start your next fight.

Turrets can randomly gain a free shot on their rise to meet you, instead of having the usual fixed delay before they are allowed to fire at you. I believe this may be due to the aforementioned problem of enemies accidentally being given wall hacks.

It is possible to buffer actions during level load screens. Be careful not to buffer the E key, as this can cause you to drop your item at the start of the next level.

The check for enemies on the level exit hatch appears to be solely based on distance, not by which room the enemy spawns in, though my observation could also be incorrect due to snakeworm jank.

Explosions linger for at least a dozen or two frames, but can only hit each entity on the map one time before it will permanently ignore that entity. This also makes it possible for you to chain reaction every grenade in your inventory, by shooting the first grenade and then subsequently pulling out further grenades and letting the last grenade's blast detonate it. This appears to be the fastest method to reset, though I have not timed it.

In addition to walls and tall grass, turrets and turret husks also seemed to act as line-of-sight blockers in my recent playthroughs.