Comment by ayhanfuat

It's a way of saying twice as fast and twice as slow have equal effect on opposite sides. If your baseline is 10 seconds, one benchmark takes 5 seconds, and another one takes 20 seconds then the geometric mean gives you 10 seconds as the result because they cancel each other. The arithmetic mean would treat it differently because in absolute terms 10 seconds slow down is bigger than 5 seconds speedup. But that is not fair for speedups because the absolute speedup you can reach is at most 10 seconds but slow down has no limits.