In general, if you want better performance, lower cas latency is
better.
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CAS and RAS Latency are two ways of measuring speed
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CAS (column access strobe) Latency and RAS (row access strobe)
Latency
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An ad for memory might sometimes give the CAS Latency value within a series of timing numbers, such as 5-5-5-15. The first value is the CAS latency, which would mean that in this case, this module is CL5. The second value is RAS latency.