I'm 99% certain that it doesn't reduce performance.
And you can see what it does by moving it close to a surface. With a normal light, the brightness decreases as tou get closer to the edge of it's raduis, so near the light actor the light given off is very bright, as you go further away from it the light brightness decreases steadily till there is no light given off at all.
With non-incidence, the light is very bright near the light actor again, but as you move further away, the brightness does not decrease at all, or at least very little. Then when you reach the edge of the light's radius the brightness drops quickly.
It's great for lighting up light fixtures instead of using Unlit which looks ghey. Just move the light close the the surface, set raduis to about 2-4 and set brightness quite high, then set it to non-incidence. If you use it for large raduis light it makes it looks all washed out and zone-lighty as most of the surfaces that are affected by it get lit up the same amout.