Would adaptive sampling be faster if the noise threshold adaptive sampling uses could be based on perceptual noise. For example, I just rendered a dark blue and white tent. Inside the tent where lighting is mostly indirect, the dark blue areas, although most likely as noisy as the white area, looked perfectly acceptable, whereas the white area still looked very noisy.
Maybe it would be beneficial to have the adaptive sampling bare this in mind, having a variable noise threshold based on material colour? Maybe even have it work on a per pass basis rather than on the combined…sort of like an automatic branched path tracing, throwing sample only at passes that need it.