It's because the stronger the forces and the higher the velocity in a cell of the simulation, the more iterations the numerical integrator needs to perform to m

It's because the stronger the forces and the higher the velocity in a cell of the simulation, the more iterations the numerical integrator needs to perform to maintain acceptable accuracy. It's obvious, really. ;) Also the universe isn't actually expanding and the red shift we see in distant stars is just a numerical error due to the enormous number of steps between here and there. :)