AI in a game is one of the most CPU intensive parts of any game, think of it if you will for graphics and textures you can push that onto the GPU. For AI its algorithms and code so its more CPU intensive and to be fair they havent majorly changed in the last decade yes we have more cores but not any significant changes like the early 2000's where speeds were going through the roof and we went through several iterations of processors every year or so with K5/K6 and the like.
Back to the original point if the AI code is written poorly there is no fixing bad code the best way in my experience is to use the state engine style where the NPC's look around and if they notice something they know shouldnt be there they investigate. Think of it as you will if you plan everything out for the NPC the game is running loads and loads of code to get them to move from x to y all the game needs to do is make sure they are moving in the chosen direction and then work out from y where to go next. Pathnoding could be as simple as a load of key points on a path. If the NPC is a security person and spots something they shouldnt (rather than having UBER sight and shooting you instantly) they could have a spot check depending on lighting levels and range so if your in the shadows there is a harder time spotting you but if your running around in light your fair game, this is how Thief done it back in the day to great effect.
There are loads of optimisations that can be done to make the AI more fun to play with but to do this sometimes as "Old duck" has said you need to rewrite the drawing board
Back to the original point if the AI code is written poorly there is no fixing bad code the best way in my experience is to use the state engine style where the NPC's look around and if they notice something they know shouldnt be there they investigate. Think of it as you will if you plan everything out for the NPC the game is running loads and loads of code to get them to move from x to y all the game needs to do is make sure they are moving in the chosen direction and then work out from y where to go next. Pathnoding could be as simple as a load of key points on a path. If the NPC is a security person and spots something they shouldnt (rather than having UBER sight and shooting you instantly) they could have a spot check depending on lighting levels and range so if your in the shadows there is a harder time spotting you but if your running around in light your fair game, this is how Thief done it back in the day to great effect.
There are loads of optimisations that can be done to make the AI more fun to play with but to do this sometimes as "Old duck" has said you need to rewrite the drawing board