Noticed that there are no lags on any other small levels:
For me it's 44% screen size, every other graphic setting is disabled - still occasional freezes in campaign world
100% screen size, all kinds of graphic settings - no freezes on tutorial/debug/etc levels
Of course smaller levels have much less objects, but it's GPU that spikes up in main world, not CPU. My bare guess is that game renders many things that potentially could be unrendered (things you can't see, things that are far away, etc)