The latency of RAM is measured by relativity to Clock speed. CL17 means it has a 17-cycle delay. This means that CL0 is the fastest there can be when there is 0-cycle delay. This means a lot less these days because clock speeds of RAM are much faster then they were before. So a CL7 RAM with clock speed of 4800MHz will perform better than a CL4 RAM with clock speed 1000MHz