Agner`s CPU blog

Note that only the Xeon E5 v3 parts have the differentiated frequencies for 256-bit operation, so I would not be surprised if the "client" Haswell parts did not show this behavior.

I repeated these tests on a Sandy Bridge (Xeon E5-2680) and found the same "half-speed" operation that you reported earlier. In my experiments the "half-speed" operation lasted for up to a few thousand cycles, but the transition to full speed operation incurred no stall cycles. It also appears that the "full speed" mode of operation is not retained as long -- even short (much less than 1 millisecond) periods of not using the 256-bit registers resulted in switching back to the slower mode.