I was experimenting with axes and pickaxes with Stone Brick Slabs, and I found out that the Iron Axe mines the same speed as the Iron Pickaxe except you do not get the block back with the axe. I did the same thing with:
Stone Slabs
Cobblestone Slabs
Quartz Slabs and
Brick Slabs.
It turned out to be all the same speeds for Axes and Pickaxes.

Some info:

It has happened since release 1.5.1 with Java Runtime Environment 1.7.0_15
It is all over YouTube
Also, it happens with wooden slabs with Pickaxes, too, but I am unsure of that.