Thanks guys. My fault for being an idiot.
So, I've come across something that is an actual problem for me. It has to do with Layer 5, or "Back Tile General". I'm editing my No Fly Zone level so it can look better for when I upload my chapter. But anyways, when I filled the back of the level with tiles, it lagged everything and made things buggy. I couldn't wall jump (he'd just hit the wall and slide down), and things were slow. Now, my PC does suck, but I've been able to play the game fine in -ultralowdetail (thanks guys for making that!). I didn't have any anti-spyware or firewalls going on, Aero was turned off, and I was even using Game Booster. But to no avail. It wasn't until after I removed the background wall that things started running smoothly again when I tested the level again.
So hopefully, one of two things is happening that you can help me out with:
1. Some kind of bug.
2. I'm doing something wrong.
If not and it's just my compy being a dick, I'm gonna have to design levels without layer decoration. Sorry.
Well, I could be wrong, but everything I've gained in my experience editing for SMB says that it's just your computer being slow. I actually had some similar experiences when I was trying to use a laptop to edit levels that was way too slow to be useful. I was having issues specifically with background tiles just screwing up all kinds of things. I've seen a bunch of patterns of bugs that are just generally occurring while in low FPS moments... and this just seems like exactly that to me. Sorry to give you potentially bad news if you have no other means to run the game, but on top of you just having those issues, it's been my experience too that running the editor in ultra-low or low will make some objects just not work correctly as well, further making your life difficult.
I wish you luck and hope you can still get your levels to shine regardless.