If light acted instantaneously you would have to calculate the effect of it's rays everywhere all at once, which I think is quite expensive given how vast space

If light acted instantaneously you would have to calculate the effect of it's rays everywhere all at once, which I think is quite expensive given how vast space is. However, since the speed of light is miniscule compared to the size of the universe you can ignore all but the most local interactions, and just schedule a computation sometime in the future when you know that the light vector will interact with something. While instant calculations would perhaps make for a simpler system conceptually, the speed limit and locality principle ensures that less processing power is needed (at the cost of a lot of memory).