I think you mean "tightening" the CAS latency (Cl)? CAS Latency = the number of clock cycles it takes before data starts to flow after a command is received. Low CAS latency at a given frequency is faster than high CAS latency at that same frequency. So if you're memory is capable of running faster than CL3 stabily, you should try tightening it down to 2.5 or maybe even 2 depending on how good it is. Tightening this timing does usually mean that you won't be able to clock the memory quite as high as you could if the timings were a bit looser. However, raw speed with looser timings doesn't necesarily mean faster. RAM can be faster with a lower frequency and tighter timings than a set with a higher frequency and looser timings. You need to find the optimum midpoint for your RAM, the point in which you get decent clocks and tight timings.