Agner`s CPU blog

The uop count is a matter of how the engineers chose to implement the feature. If the implementation is internal to the functional unit, then it would not require extra uops, and I do not see any significant change in uop counts between the "slow" and "normal" phases. (The uop counts are elevated for the iteration that includes the transition, but it is not at all clear what is happening in that step.)

I think I forgot to mention that, just as you noticed on Sandy Bridge, there are no "warm-up" effects when using scalar AVX operations or 128-bit SSE operations. (I did not check 128-bit AVX, but there is not a lot of reason for that to be different than 128-bit SSE.) My assumption that the data is running though the "lower" 128-bit pipe is based in part on the observation that the 128-bit pipeline is available at full speed at all times. From an implementation perspective running the 256-bit operations at a lower frequency does not make a lot of sense when there is a full-speed 128-bit pipeline ready to use.