Why do register machines outperform stack machines?
I think the prime reason why register machines are better at performance because registers are within the CPU chip itself, however stack is generally the part of main memory, accessing register is on average 1-3 clock cycles, level 1 cache requires nearly 10-20 cycles on average, and depending on the design it could have level 2 shared cache which takes about 40 cycles on average, finally main memory takes 100s of cycles in comparison per initial word access then once you access it, the further block containing the required word is only incrementally cycle consuming. Hence register based architecture should always out perform stack based.

The data presented is definitely not according to today's state of the art processors take it from more of a conceptual stand point rather than practical one, and that I believe was the intent of the question.