Just ignore any actors which are far away from the player(s). Don't update them and don't allow active actors to interact with them.
Most games divide the game world into zones, and only update the actors in the zones the player is in and the adjacent zones. Many even despawn the actors and respawn them at their initial positions when the zone becomes active again to save memory.
When it is important for your game concept that certain actors do things while the player is away (for most games it is not), you could freeze the action in inactive zones, and when the player comes near calculate the activity in the zone which happened in the meantime using a simplified model. When you have, for example, an RPG with a blacksmith NPC who makes swords by automatically collecting resources and bringing them to his forge, the simplified model would not do the routefinding but just use an estimation how many swords he would have made during the time the player was away, remove the required resources from the surrounding and give him that number of swords.
When you have actors which travel around the whole game world and you want them to keep doing that even when the player isn't close (like in Oblivion or Skyrim), you could strongly simplify the route-finding while the player is away. When you know by experience that traveling from town A to town B takes about 20 minutes, you can treat this as a single step with a cost of 20 minutes in your route-finding system. When the player now suddenly enters the area between town A and town B, and the NPC left town A 15 minutes ago, you would calculate the actual path and spawn the NPC on 3/4 of its way towards town B.