Comment by formerly_proven

Yuo see, when standard memcpy runs it has a big variance of power, i.e. large cycle to cycle differences in effective current through the processing chip. Agner Fog's memcpy is optimized to use the pipelines effectively, makes the processing operate smoothly and not in discrete steps, much less variation. That variation causes fluctuation in primary voltage supply which leaks into the analog path, creating noise and harshness.