Original file 1Mb

GammaSlamma - 909kb - 12.4 seconds
PNGcrusher - 913kb - 49.6 seconds
PNGenie - 803kb - 119 seconds

So for the extra 10% or so file space reduction, you're having to wait TEN TIMES longer with PNGenie.

PNGcrusher is also useful if you want everything stripped APART from the gamma.