[BeeOnRope]

Unless the AMD design is unusual it is not very close to zero return: a significant part of the "path to memory" involves things run at the core clock, in particular everything from the core to the L2 and probably some part of the coordination logic which communicates with the "uncore". I'm not sure about AMD chips, but on some chips there is a relationship between the uncore speed and the core speed: e.g., the uncore speed might often be the same as the maximum core speed for any core on the socket.

Adding to that, there are other effects that allow core frequency to leak into the performance of memory-bound programs, such as a higher frequency allowing the core run ahead more quickly to get more memory requests in flight, recover more quickly after a branch misprediction, etc. Try it sometime: find something which is really memory bound and crank the frequency way down: there will probably be a significant effect, but not nearly in proportion to the frequency difference.