mystery tower is by far the lagiest map, since it uses all possible positive Z levels. The original game maxed at 3 z levels, the towers i think 15. Space maps have more(due to + and - levels) but they dont have floor terrain to render. And not much other terrain either. Uniform voids are easy on the engine.
The taller the map the more voxels are rendered and tracked at once. And truth be told the engine is not especially well optimized for its maximum limits, and we are compensating for these inefficient processes with modern hardware power.