The reason why the picture that has Local Reflections on is so much brighter than the one thats off is because for each pixel on the screen, a single ray is casted to receive the reflection color. All of the white and light blue colors on the snow are reflecting onto every other pixel in range of the texture so everything seems brighter. If you can see the white lines in the little river in this map, thats the reflections really in action. The water is actually reflecting the light/colors on the river. I didn't see any FPS drop, maybe 1 frame, so I would turn it on.