The idea that all neighbors move to the next tick simultaneously is a fundamental assumption in cellular automata in general. If you try changing that, the optimizations that allow us to simulate CAs at any kind of reasonable speed ... all stop working, pretty much. It's kind of painful even to think about.

Which means there are probably very interesting rules out there somewhere, where CAs run faster/slower depending on pattern density -- it's just going to be very tricky to explore that particular search space.