How efficient is Terrain.SampleHeight?

Hi,

I want to use the terrain engine for a mobile game and I'm wondering how efficient it is. I also want to know how efficient Terrain.SampleHeight is? Can I use it on a per update basis? Does it use ray casts or is the data already stored?

Also, on a side note, will reducing the poly-count of the terrain increase performance, and if so is there a way to adjust the poly-count of the terrain?

Thanks!