I suspect that the drop in wind speed IS the result of turbulence. If energy is extracted by the windmill the total energy of the moving air has to decrease somehow, according to conservation of energy and entropy has to increase. The increased entropy is likely what we see visualized in part. That would be the condensation of water previously present as water vapor as a result of a pressure transient in the vicinity of the blade.

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."