Nexus 5 CPU is an ARM architecture, but it can't have exactly the same specs because it's an emulator and performance depends on your computer. x86 emulator is generally faster than ARM due to PC x86 CPU architecture.
Skin field is just emulator window appareance/size, you can enable/disable control buttons and define a window size which does not depend on the AVD's resolution (useful for non FullHD computer resolution).
EDIt : x86 emulator is not faster on every computer (depends on CPU), ARM is faster on some computer because ARM instructions are lighter than x86.