the linux kernel has this to say
Code:
CONFIG_M586: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *x *
*x * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * x *
*x Select this for an 586 or 686 series processor such as the AMD K5, * * *x *
*x the Intel 5x86 or 6x86, or the Intel 6x86MX. *This choice does not * * *x *
*x assume the RDTSC (Read Time Stamp Counter) instruction.
So it's probably faster it just lacks some instructions (no game uses them though)