Comment by Symmetry

Well, all those instruction prefixes mean that decoding x86 instructions is really hard. That leads to more cycles to decode, for a larger branch mis-predict penalty. And your decode unit takes as much power as your integer cluster (which is still small potatoes compared to all the OoO resources). And you're limited to 1 decoded instruction per cycle when executing instructions brought into cache for the first time, before the processor can tag the boundaries (but again, most instructions are executed many times).