The best predictor of air drag is the barometer reading. This correctly measures air density, factoring temperature, humidity (which lowers density, which is why a low barometer is a predictor of rain), and altitude.

So winter or summer, we live in a world where the normal barometer range is between 29 and 31" of mercury (at sea level), and can see swings in drag in the 10% range.

Once the increased drag from higher density is factored, that percentage difference remains the same at all speeds. However, a 10% difference in drag becomes more meaningful with increased speed because air drag is a larger percentage of total drag.