The above guides are great, I just wanted to clear up a few things and give some tips that make this a lot more manageable.
Firstly do this on pc or the newest xbox you have, I started the game on my original xbox one but the load times and frame rate made me switch to my pc to save the hassle. It's definitely still possible on old xbox ones but more annoying.
You don't have to bother with an external speedrun timer unless you really wanna setup splits as goals for each section, there's a built in timer that you can turn on at the bottom of gameplay settings that'll appear in the top left every time you start a new game. The timer starts when you walk through the first doorway in Introduction and ends when you hit the clock in Retrospective. The timer also pauses when you pause the game so feel free to do so whenever you need a break or check something for an upcoming section.
I got this achievement on stream with a time just over 18 minutes, but that was using some risky/hard tricks for certain sections. I made the video below as an easier guide to follow and it leaves plenty of room for error if you lose time in an area.
Now I'll go over some tricks and if they're worth doing or not.
Chapter 2: I do the trick that CraigTMG talks about in his guide at 3:44, you can practice this but in my opinion it doesn't save much time so you can just grab the second cube as normal.
Chapter 3: I grab the side of a die and place it vertically to get past the puzzle at 7:06, I don't recommend doing this as it saves hardly any time and it's easy to mess up. Just hold it to the ceiling, drop it until it's big enough and use it as a ramp like normal. I also had some trouble with the part right after that but you can use the cube as a step without worrying about the smaller cubes it splits into.
Chapter 4: You may have seen people do the dark section without the red light but it's not worth practicing, it only saves like 5-10 seconds and there's better skips later.
Chapter 7: This is probably the easiest and biggest skip in the game. At 18:35 you'll see me clip through the wall into the elevator that's supposed to be blocked off, for some reason it's still programmed as a normal elevator and will complete the level if you can get in. You'll want to jump on the fire extinguisher, hold forward, and look slightly to the right. You'll know when you're in the right spot because your screen will stutter a bit or you'll feel like you can't move right any more, if you're not in the right spot look and move left then slowly look right while moving forward again. When you're in the right spot you just have to hold forward and tap
jump, if you hold jump you could hit the clip weird and the momentum will send you upward and you can't get into the elevator form there. If that happens just move backwards out of the wall and retry. You have a lot of time to complete this trick before it teleports you where you're supposed to go, but if that happens before you get the clip just pause and reload checkpoint. Even with the time spent walking back to the blocked elevator you'll save more time skipping the entire level.
Chapter 8: This skip is a little harder to explain but is relatively easy, I do recommend you practice this a few times before your run though. You'll want to go through the first portal once then make it somewhat small (shown at 19:12). Once it's around the size that you first picked it up, pick it up, make the door of what you're holding face the opposite way as you and carefully jump on the ladder without putting it through the door you came out of. If you accidentally place it in the door just reload from checkpoint and retry. While you're on the ladder you'll want to jump then drop the building at the apex of your jump once you've seen it has clipped through the wall, feel free to jump a few times before dropping it to get a feel for the timing. Once you've dropped it through the wall, walk back through the door, pick it up and you'll want to find the snowy looking area that's in an oval like shape with a few wood doors around it. If you've walked around for a bit and can't find the area just reload checkpoint and try again. When you're at the right area you have to make the building you're holding pretty small then hold it against the tiny black door (shown at 19:30). This will make you the size you're supposed to be for where you're at. Now you can proceed through the chess puzzle and the last few puzzles normally or you can do the 2nd clip (shown at 19:34). For this you'll have to look at the left side of the doorframe while inside of it, hold forward, and slowly hold right until you see the area to your left is black. Once you see that you can move left and you'll fall until you reach the zone that'll end the chapter. If you've passed the black area on your left just move left and slowly move to the right again by using the steps previous.
After that, the last chapter will start and you just have to click the alarm clock to end the speedrun which will hopefully get your achievement. As long as you use the skip for chapter 7 and the first skip from chapter 8 you should save plenty of time to make the speedrun way easier.
Upvotes are much appreciated and let me know how else I could improve this guide. Happy Hunting!