You could cut lag with sections of large employables ,very large building POIs, it would be about as laggy as a planet,water is only as fast and furious as the decibles are high.
I wonder if it would be possible to add hazards that move across the surface? Like a grove of carnivorous plants, a swarm of army ants, or a "zone of corruption" that slowly moves across the surface of a planet and destroys everything in its path. It would be neat to have a spawning point that is programmed to move randomly across the surface, and scenario creators could convert any static spawner to a mobile version.
Can confirm making cities makes lots of lag On a private server we made a 'city' based on several city block pois crowded together. It made the world a bit more real, but also added plently of lag whenever you went there.
This is more possible than you think. None of the biomes have to follow any logical understanding of temperture, humidity, etc... I could easily make a desert in the mountains just as I could a jungle in a searing inferno temperture wise. Just tweak the temperture min/max, alter the biome selectors, done.