On this morning's ride I had a chance to check this out. 2x20' into and then with a 5mph wind. This is in the delta, so the roads are flat as a pancake other than climbing up and down a levee. https://www.strava.com/activities/530832682/analysis

First interval: 261w AP, 20.3mph.
Second: 262w AP, 22.7mph.

So, pretty significant speed difference for the same effort with not much wind.