Dry with air circulation. It dosen't matter at which speed you dry per se. The idea is do remove all the free water in the plant cells, (that's the water inside the plant cells) and then in curing you remove all the bound water, (that is the water between the cell walls).
Just remember that the warmer the temps, the greater capacity for the air to hold water. Relative humidity is just that. The ability of air to hold water relative to a given temperature.
The speed at which you dry your plant material will have an impact only in that some off gassing of chlorophyll, and other compounds will take time to occur, and certain metabolic changes in starches and sugars need time to take place. Of course, the more green, leafy, shit you smoke, the more like hay it's gonna taste.
If you blast your harvest with a fan, you decrease the chance of mold, but you also can falsely read your harvest as, "dry", when in reality only the outer surface is dry, and the inner portion of the plant will still need to wick moisture away to the surface to remove the free water.