What setup do you use ? Grid size ? Do you have any moving obstacle and/or regular "regenerate obstacle map" ?
Once the obstacle map is generated, paths are computed on demand, and a path asked multiple times is search only the first time (it directly triggers the OnPathFound next times).
If you have a path finding on every unit, and if every unit is itself a moving obstacle, and all move simultanously, yes, your CPU will melt. I will then melt again, and again, just to say it's not happy.
Moving obstacles in particular are a biiiiig hit on the FPS as they disable the behavior's "cache".
Knowing more on your game could help detect the problem though.