Don't think I agree but I'm not sure.
Does the magnitude of the downwash increase with speed? *Provided* the a/c is in S&L flight then L=W at any speed. L is a force & is a reaction to accelerating a mass (the air) downwards.
F=ma
The air mass flowing over the wing is constant (barring temp/press changes to the body of air) so acceleration must be constant for the force remain constant.
Maybe I'm wrong? On the other hand:
At a slower speed there is less air in a given time - but the wing is operating at a greater AoA. Maybe that means the reverse: Less mass in a given time so more acceleration to the mass is needed to maintain F.