Efficient line of sight system

Hi folks,

I need suggestions, tips, and general help with developing a system which I can apply to as many as 100 characters that allows characters to 'see' each other.

Currently I have a raycasting system that forms a 90 degree FOV cone in front of each character and works really well to detect other characters and obstacles, etc. However, even using as few raycasts as possible, this kind of system on 50+ characters has a massive impact on framerate.

I need something cheap, but effective, is there such a thing?

One thought I've had; since all the characters are navmesh agents, is there a way to tap into their detections of other agents?