So originally, fade-ins were being done by setting objects to active, setting the material's alpha to transparent, and then slowly changing that to opaque. It was probably super super costly, particularly since there were frequently about twenty objects doing that at the same time and that meant lighting couldn't be kept constant, but we kept it in because we didn't know of any alternatives.
But those dark days are behind us. Now featuring: camera tricks!
two separate cameras + culling masks + one canvas set to overlay camera
I feel like a magician pulling rabbits out of a hat.