Making it "slow" would either have to be 40% for 2 seconds or nothing at all, Yasuo has too much mobility to use a slow beneficially. Not only that, but his knockup with a CD is kinda retarded, due to the fact that Yasuo has his frustrating stuff NOT pinned on his tornado, but his regular Q's and E's + Auto's.
A "what if" situation would make him significantly gimped to fight any juggernaut(
) in late game, ESPECIALLY Dunkius.
And FYI, CC is KILL POTENTIAL. Especially Hard CC. You'd get away from that Yi if he didn't have that Red Buff Slow, right? Well he did, and it slowed you enough for him to either FLASH or Q to you.
Yasuo wouldn't of got you if his Tornado wasn't up that fast, and he caught up to you while you were knocked up next to minions, right? Kill Potential. And something like this change would need a VERY good compensation buff, like his projectile being a faster speed, instead of being so slow. As fast as Ez Q atleast.
So, basically. This change would be weird, and wouldn't solve anything either. It's in the wrong area.