I had a similar issue when I got to world 2 and beyond (since there is much more going on in the levels than in World 1).
I resolved it by adding "-lowdetail" to my launch option in Steam (I right clicked Super Meat Boy -> Properties -> Launch Options). You need to launch through the Steam Library when doing so for it to take effect.
You may also be able to do this from the Super Meat Boy Application icon on your desktop by right clicking and going to properties, although I haven't tried. It completely solved every bit of lag I was having. Give it a shot.
I love you, you are a beast, I award you +1 internet