4.5 is fast enough for the league, but it's not just the 40 that matters. the 40 doesn't account for burst, which is vital, nor does it account for "football speed".
the burst is going to come in the form of things like the shuttle, cone, etc. so a guy like abdullah gets away with his size in that way. or a guy like yeldon may be a power runner in every sense, but he's got the initial burst that matters.
i guess i'd be more with you if he played like a game breaker, but he doesn't. maybe with another year of taysom, we will see more of that and you'll be right. i hope so.