I'm comparing the CS5 & CS6 mixer brush, with moist, light mix, right now...
At first I wondered why the CS6 mixer brush was slower, until I realized that it looks like the actual brush stroke in CS6 is more advanced. The CS6 stroke looks like it's calculating the overlapping brush strokes & it's interaction with the texture more accurately to real life.
I haven't read any articles on improvements to the CS6 mixer brush. Has it actually improved or are my settings just different in CS6 from CS5? I'm just wondering, since I'll probably need to work with "Sample All Layers" unchecked when working with the CS6 mixer brush.
After playing with the mixer brush for a while in cs5 & cs6 I've come to the conclusion that cs6's mixer brush is simply slower than cs5's mixer brush when "Sample All Layers" is checked (there are only two layers in the 3000 x 2000 file). The cs5 & cs6 mixer brush gives pretty close to the same results.