Ok, some findings on the door clip thing
Can't find any other skips for E1, other then E1M4
Can't find any noticeable skips for E2
E3M2, clip through the Moon Key gate, to the room with the safe
E4M1, clip through the giant wooden door, which leads into the room with the Cultist around the table (This is quite tricky to pull off)
E4M2
The Key Eye skip is just the same skip as the doors, so just do the same thing.
E4M3, you can clip through the grey door on the right, when you exit the water (This is Armaaaa's finding)
The E4M4 clip mentioned by hotmultimedia, as well as the E4M6 one
E6M2, clip through the gates with the fire key behind it
E6M3, clip through the big wooden door near the start (leads to the room with the E1 boss and the door with the level exit)
E6M4, clip through the fenced gate, so you don't need to restore the power
These are the ones found so far, many of them saving a few seconds (expect for E1M4, E4M2 and E4M6, which can save a lot of time)
Still nothing found that can cut E1M6 and E6M7 time down massively.
Can't find any other skips for E1, other then E1M4
Can't find any noticeable skips for E2
E3M2, clip through the Moon Key gate, to the room with the safe
E4M1, clip through the giant wooden door, which leads into the room with the Cultist around the table (This is quite tricky to pull off)
E4M2
The Key Eye skip is just the same skip as the doors, so just do the same thing.
E4M3, you can clip through the grey door on the right, when you exit the water (This is Armaaaa's finding)
The E4M4 clip mentioned by hotmultimedia, as well as the E4M6 one
E6M2, clip through the gates with the fire key behind it
E6M3, clip through the big wooden door near the start (leads to the room with the E1 boss and the door with the level exit)
E6M4, clip through the fenced gate, so you don't need to restore the power
These are the ones found so far, many of them saving a few seconds (expect for E1M4, E4M2 and E4M6, which can save a lot of time)
Still nothing found that can cut E1M6 and E6M7 time down massively.