My possession receiver will have 90 speed and 80 agility by level 48.
And he'll still catch the ball 80% of the time.
I don't see why people imagine possession being slow.
If you look at the NFL, "possession" type receivers are still VERY fast.
I really could not imagine a person running say a 6 second 40, which is what yours would do right now (or slower) and still be able to do well in professional football. He'd have someone consistently in his face, and what's the appeal to throw to you at that point? QBs want to hit an open man.
Then say for some reason the QB does decide to throw to the guy who's heavily covered. What's the guarantee that since the CB is RIGHT THERE ON TOP OF YOU, probably even IN FRONT of you, that he doesn't grab it before you ever get a chance to, or swat it away?
And what if the throw isn't perfect? It's 5 yards to the right and it's a fast throw because the QB was under hurry pressure? You don't have the ability to adjust to the throw because you're slow and can't turn.
Agility is one of, if not the largest factor in being a possession receiver.