I may be wrong here so if anyone knows better feel free to correct me. But the timings signifiy how many clock cycles occur before an operation occurs. So CL5 does whatever it needs to do every 5 clock cycles, while CL6 takes 6 cycles. So 5 is faster by ~20% theoretically. But I think in practice you don't see anywhere near that performance increase. I expect the increase between CL5 and CL6 is probably much closer to about 2% or less.