Many people are complaining about optimization, and even more after trying the -b patch version (they say performance got worse and lag/freezes have increased, compared to main version)
Viewing post in Voices of the Void 0.8.2 feedback
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)
Most things do seem to have proper culling, you can see objects or entire chunks of the map unrendered for a split second if you whip your camera around or open a door.
Distant object rendering is decent too, low view distances hides pretty much everything far away if it isn't directly a part of the map or something giant that you should obviously see.