They aren’t negligibly slow. Hindenburg had a cruise speed of 120kph, certainly a speed where you optimise the aero design for going forwards. Some new designs

They aren’t negligibly slow. Hindenburg had a cruise speed of 120kph, certainly a speed where you optimise the aero design for going forwards. Some new designs of airships shape the hull as an airfoil, so it generates lift, supplementing the lighter-than-air-ness.