Assuming that during the middle ages on Earth, all wind stopped. Some magic spell has made it so that air will no longer move faster than a gentle breeze from thermodynamics (the speed air moves out of the way of a solid object is unchanged). What would the be the consequences of this?
Short Answer: Everyone would die
I don't think there would be many dramatic instant effects. Some children (and adults) would have their kites drop from the sky, stuff would stop blowing around, sailing ships would be stuck and the temperature of some room would increase or decrease slightly. After some time the last wave would hit shore leaving the sea pretty much still, tornadoes and hurricanes would stop.
After a few days the problem arrives. Without wind systems warm, moist air won't move around. Water might still evaporate but it won't travel meaning anywhere away from a large body of water will dry up. Lakes might be alright as the evaporating water will fall back into them and the sea will be fine but anywhere else will get very dry very quickly. Plants, animals and humans will slowly die from dehydration with most water inaccessible as salt water or ice. Even if we survive this the problems don't stop. The equator will no longer shed its heat as quickly so it will become very warm, mountains may remain cool enough to be habitable but nowhere else will. At the same time the poles, deprived of heat from the equator, will freeze solid.
There are other effects but it basically boils down to everyone dying.
Temperature extremes would increase, probably threatening life near the equator and the poles.
Travel by sail would disappear -- and any oceanic sailors far from land when this happened would be in real trouble!
Fire would act differently, without convective air currents to move the heat, bring in more O2; probably not able to spread as fast, but hotter and more asphyxiating where it does burn. Perhaps only small fires are safe/usable while this spell is in place.
General situation would be "as if" air viscosity would be much higher.
This means air would still travel for shorter distances (at the maximum 1mph speed), so a (relatively) narrow strip near the sea would still be habitable.
Coasts relying on breeze would have very narrow (<<10Mi) strips, while geographic winds like Monsoon could penetrate much farther.
It would be interesting to see if the energy is dissipated by viscosity (converted to heat) or kept as kinetic (air becomes more heavy and thus the slow moving air would still push as if a molasses wave). That depends, of course, on specifics of the "spell".