There's a lot of NPCs in NPP and mutants ditto. Probably the most than anywhere in the game: military, Monolithians, zombies of different types some armed, skeletons, mutant insects, some of the monolithians are invisible, things blow up, then there is the intoning voice, and other sounds including the geiger counter type noise. Also MO needs plenty of protection in suit, artefacts, weaponry.
A lot goes on outside, then down the drain cover, and immediate zombie attacks. The construction of the complex architecture also needs to be loaded, lastly the ejection of MO via the Wishgranter and a complex, second run through warps.
Even a very good pc like yours may struggle. Regards, TrueB.