I don't remember ever experiencing loss of airspeed when turning downwind. I was taught to maintain airspeed throughout the turn. In shallow angle of bank turns, airspeed usually reduces slightly because of the increased back pressure on the stick required to maintain altitude. In steep turns, the airspeed is maintained by applying power to overcome the back pressure on the stick.
I understand the loss of airspeed in an aircraft like a Harrier (which I have not flown) or helicopter (which I have) in the hover, but I believe that in other aircraft, in forward flight, the pilot should control the airspeed by using the stick. I don't recall, in another life, having airspeed changes imposed on me when flying holding patterns into and out of wind.
I also note in my old copy of "Flying Instructor's Handbook - Piston Provost" when teaching low flying, we were to "Fly into wind and turn accurately through 180 degrees. Fly downwind then turn accurately through 180 degrees. Maintain constant airspeed. Demonstrate in moderate wind conditions."
Have I the wrong end of the stick in this discussion? Is the loss of airspeed/downwind turn a theoretical rather than practical question?