Collisions simulation
Ever tried simulating collisions between numerous objects? Or calculating the behaviour of 3 mutually gravitating objects? You can't do it really accurately for an extended time.
The reason is that when one object moves, you have to check all the other objects to see if they have collided with it. At the same time you have to increment their positions, and check if they have collided with any of the other objects themselves. Can't be done without enormous computing power.
So, how does our universe do it? Fundamental question.