My impression is that the tax is also considerd to be due to the fact that the ISA has been carried around for many , many years and the compatibility forces compromises on the decoder implementations. The ISA could be considered bloated by some. I certainly missed any objective ISA analysis / comparison if ever there was one recently The last official context i recall it being tangetially refered to was the 64 bit x86 introduction, with Intel invoking the need for a clean slate when Itanium was spawned. And then in reply, AMD came with amd64 , a port which according to them incured a much lower performance tax than Intel had anticipated (something in the vein of ~ 5 % , ofcourse pulling this number from ancient and unreliable memory) Micro-ops' existance, their 'fission' / fusion and whatnot are implementation decisions. They could get abandoned if they would not be worth it. So that shouldn't be a strong reason to support the 'tax'