I'm not convinced it actually ever did anything to improve performance. In fact on some systems I think not clearing the background is a de-optimisation, because the GPU has to use less efficient rendering methods if it thinks the previous frame's contents could appear on the screen, and a clear on startup proves that won't happen. Anyway, the C3 runtime now automatically works out the best place to clear. I think that should never have been a user-exposed option, really.