For whatever reason, as soon as I left the Sprout Tower after beating it, the game started lagging extremely bad-but not the menus. The only problem I have is that the controls don't respond to anything, and the only way to get to the menu is to Alt+Tab to tab out, forcing the game to pause. This is nowhere else in the game(at least, as far as you can get after beating Sprout Tower).
Edit: OK, it's at any area the game considers outside or a forest. Caves and buildings are just fine.
Edit: OK, it's at any area the game considers outside or a forest. Caves and buildings are just fine.