How are you lighting the scene? I suspect you are using more than one light with ray-traced shadows, or an environment light with Ambient Occlusion on, or both. Raytracing and transmapped hair really slow the system down. One solution you might look at is using uberSurface on the hair, since that allows you to tell it not to respond to ray-traced shadows or occlusion: Whether the result is acceptable or not you will have to decide for yourself.