The main technique is to simply not run the pathfinder every frame. Typically, you only need to run it once a second or less to get good results. Depending on what you're using it for, you may even just need to run it once ever.
Another common technique, when pathfinding for multiple units in different places to the same end location, is to instead search outwards from the end location to each of the units. This requires only a single pathfinding call for all the units, rather than one for each unit. This requires you to actually get down into the pathfinder-implementation, though; if your library does not already support this technique, there is no way for you to "fake" it, you'd have to switch libraries or write your own pathfinding code.
I can also second the recommendation for Aron Granberg's Unity pathfinding library; I've looked at his code and actually spoken with him personally, and I can vouch that the library is very well-optimized. I am not familiar at all with RAIN.
If you do decide to go the code-your-own-pathfinder route, I have written a priority queue optimized specifically for pathfinding that would be useful. I have not tested it with Mono/Unity, but I believe it should work fine.