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There's a number of tech solutions to scaling mob AI. Most of these are also applicable to non-online games as well:

- Running AI at a different frequency than the game. The network event loop might run at 30Hz (33ms tick delta) but AI doesn't need to make decisions that often, maybe every 100ms or 500ms etc.

- Separate decision making from performance. A cheap execution system can run at a high frequency, while an expensive decision making system runs at a lower one.

- AI LOD. Mobs that are far away run much simpler AI that doesn't try to perform detailed animations etc. Mobs that are in unpopulated areas get frozen, and thawed once somebody enters.

- In instanced dungeons, mobs don't even exist until the players enter, and once players leave, the mobs just get unceremoniously killed.

- Offloading. Mobs are just game agents so in online games it's possible to offload them to other servers that get load balancer as players move about, areas get frozen etc.

Basically the big question is: if nobody is interacting with a mob, does it need to exist? In most online games they do not. And even in sim heavy games, maybe they don't need to run the same high fidelity AI when nobody is watching.

Then the last tip is:

- Make the AI as cheap as possible for the task at hand, but no cheaper. A mob in MMO that is all aggro and easy to kite doesn't need to have complex strategic cognition, and it would probably be a detriment. Everything need a to match it's context.

Source: I make games. :)

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