Desktop parts have no dedicated AVX clock (however, their base vs. all-core turbo pretty much does the same thing), while server SKUs (Socket R3 and later) basically have two distinct clocking ladders, one for non-AVX code and one for AVX code (the "mode switch" quantum is rather coarse, on the order of one millisecond). So both base clock and maximum turbo clocks are lower in AVX code compared to non-AVX code.

As usual, actual achieved clock speeds depend on thermal performance and are not fixed. If cooling is completely insufficient, the clock frequency will drop below base clocks, i.e. thermal throttling.

To clarify: This isn't a secret, but part of the specs and even in some Intel slides. (However, platform and processor specs, even at a basic (product D/S) level, tend to be a rather lengthy read, so most do away with reading all that).