Simply put, they are slower.
Long explination, for gaming AMD is slower because they have a slower cycle per clock, per core. Essentially AMD's module system is kind of like Intel's Hyper Threading technology, except they do the thread splitting at a hardware level where a single module acts as 2 cores. Because of this single threaded apps run slower on AMD FX processors, but IPC is also slower so raw Ghz doesn't make up for the speed. In other words, there is no need for more than 4 cores for gaming when theoretically the max a game can use today is 4 as it is. Most only use 2.
Considering this is an Ivy Bridge processor it makes it that much faster than the AMD FX line for gaming.