Most efficient "find" operation.

I'm creating a traffic simulation type program, and I'm trying to determine what would be the most cost efficient method of calculating a route using waypoints. The catch is that the user must be able to place/build the road system. The pathfinding must be able to compare many positions and distances/angles efficiently, but there will be time for coroutine/delay while the route is being calculated.

I've narrowed down my options to either overlapsphere, raycast "scanning" , or findobjectswithtag. If there are no better options, which of these would be considered to be the best in terms of efficiency?

Edit:

I should mention that I plan to use layers and tags to determine which waypoints are "suitable progress", and I'm only wondering about the efficiency of "getting" the point data.

 

Re FindObjectsWithTag: this operation is slow (it's not table-based; it will walk through all tagged objects for every search).