Simulating a large world seems like a textbook example of an "embarrassingly parallel" problem, doesn't it? At least that's true for cellular automata.
I think the problem comes from that the world is a large mass of interconnected mutable state. You don't know if updating a particular object will update another.
Think of a player crouching on a plank that another player just phasered. If you continue simulate the first player's movement "crawl forward" by itself, how do you integrate the result of the plank being disintegrated, causing the player to fall? The first player's simulation outcome depends on multiple inputs, but they aren't findable directly from that player's perspective. You have to first simulate the phaser beam to know the plank is gone to know the player is falling now, not crawling.
And then imagine that with far more complex rules and a few hundred thousand objects having similar interactions, each one possibly modifying any other one.
