Five IL levels on Serious difficulty.

Sand Canyons
Who knew that this level would eventually only take 2 seconds longer to finish than Hatshepsut? Only a single enemy is seen during the level which is a record. Damage boosts are severely increased on Serious difficulty, something I take advantage of to get through a pretty specific spot in the teleporter perimeter, which, unsurprisingly, teleports you back to the level upon touching it. From there on it's all just about running out of bounds to the exit trigger. Since you skip the whole level, this run is 1 minute and 10 seconds faster than stx-Vile's run.

Valley Of The Kings
This is basically the same trick used in Sand Canyons. Killing the Gnaar spawns the Kamikaze, so you have to time it right, but the actual boost is easy. This level is really long and painful when played normally. Even with the large skips used in stx-Vile's run, going OoB saves 2 minutes and 21 seconds.

Moon Mountains
There is a secret armor around the fourth platform that is catapulted up when the minigun ammo is grabbed. I found out that there's actually an invisible jump pad that catapults it, so by holding jump when landing on it you get thrown really high into the air while still able to control the direction. How fitting that you can then simply jump out of bounds, no teleporter fields and all, since the developers obviously thought no one could ever get up there. This shortcut saves 36 seconds.

Oasis
Inside the secret cave (the classic and oh so secret behind-the-waterfall-secret) there's a teleporter field that saves me a lot of running by taking me closer to the entrance. Thankfully I don't have to worry much about the Adult Arachnid this way either. When looking in the map editor, I found a very small spot that is not covered by the teleporter perimeter, and as you can see I'm rocket jumping right into it. This took many, many attempts since you have to slide in a certain way to get through the field, but saves a massive 2 minutes and 36 seconds due to the timed Marsh Hopper fight normally encountered.

Luxor
This is basically an OoB version of stx-Vile's run. After getting on top of the perimeter of the level by doing a few nice rocket jumps, TheVoiid eventually runs into an OoB spot which takes him all the way to the exit instead of falling back into the level. This lets you skip the lengthy final arena battle and saves 3 minutes and 24 seconds in total.