The speed in Hz (MHz being 1 million Hz) defines the number of cycles per seconds, but the timing defines the number of cycles it takes to do specific "operations". So memory that runs faster but with higher timing might nullify the benefit. For example, DDR3 right now runs up to 100% faster (DDR3-1600), but often with 125% higher timing (CAS 9 vs CAS 4) and the gain is minimal.