Axes break stone slabs at a fast speed

When hitting stone slabs (possibly only doubleslabs) with a hacked axe with Efficiency X, I noticed I was able to break them quickly. I know this is not the efficiency because I have a shovel with the same thing and it breaks them at a normal speed. This was tested on a Minecraft 1.5.1 Bukkit server. Also causes a small lag spike sometimes, as indicated by my FPS.