The game runs better for me on PC (lowdetail) than on Xbox360, and my PC is a 5 year old laptop that sucks. So even if the game's programming was optimized for the Xbox360, the programming is still lacking a couple of things. This doesn't take away from the fact that it has very good controls and a lot of levels in a fairly small (300MB?) package.
Do you ever experience any sudden lag spikes (preferably in the same spot on a certain level)? Do you ever get control glitches, clipping, or game crashes. These things (contrary to what Team Meat will admit) ARE NOT dependent on hardware/software/setting 90% of the time. The only thing I've experienced that runs better on any PC would be the loading times, and flipping between World screens...other than that the 360 version is rock solid.
But of course we can't forget the 360 version has it's share of glitches, control anomalies, etc. Not to mention the retarded fan/gravity pulse physics, terrible hitboxes (which on PC can glitch out and kill you).
But I digress, SMB was fun while it lasted but I just wish a better developer who's known for absolute polish and features that work DAY 1 (like Retro Studios) would have made SMB instead. The community wouldn't be dead already if the technical issues were near non-existent and it was superbly polished.
My SMB Post Mortem: Amazing game with great platforming ideas and controls, but utterly failed execution when it comes to polish and delivering a product the community can stand behind for years.
I apologize for going off topic, try to eliminate Steam/firewall/AV and see if closing all sorts of apps can diagnose the issue.