I'm also getting this exact same problem. I noticed there was a SMB update on Steam and I decided to give it a spin and the game freezes every time I complete a level. I even tried rebooting my PC but to no avail. I'm roughly 3/4 of the way through Chapter 4, and it's happened on every level i've tried, even when I try replaying levels.
I did notice that the occasional minor lag or hiccup I had when playing the game before has completely disappeared, so that was nice to see. Hopefully there can be a quick update to fix this new problem, because it's making the game kinda unplayable at the moment.