I’m pretty God-damn sure that they don’t cause lag considering I’ve had at least 4 structures in the 75,000 120,000 block range each made entirely of Black Ice, using foundations, walls, ceilings, pillars, ramps, stairs, roofs, crenellations, fences, and fence foundations. As well as building in ways you normally cannot due to LBPR settings. No server impact.
I’ve even seen players with lower end systems (such as GTX 780s and 1400s) not experience anymore slow down compared to other large structures.
But congrats to anyone who thinks otherwise. You’re a card carrying member of the Hyborian Flat Earth Society. 
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