How to check constantly check for the closest player?

I’m planning to make a game where NPCs engage into combat with other players if they get to a close enough radius, which I believe would work through having a loop using the DistanceFromCharacter function and checking every single player and see if they are in line of sight with the ray cast function.

Games like apocalypse rising seem to have LOADS of NPCs with follow you if you get into their line of sight and still run well. There must be an efficient way to do this, help?