Hmmm, he ran a 4.6 at the combine and didn't try to beat it at his pro day, which suggests to me that was a very good result for him . Saw a short interview with him at his pro day where he seemed really happy with his 4.6.
I know getting separation is a slightly de-emphasised advantage in a WCO, but it's still pretty important, not to mention stretching the field. A lack of pace is a pretty debilitating weakness for a WR in any offense.
At the Shrine game, I saw comments that he struggled with quick snappy passes as you would expect him to face in a WCO, but was a stud plucking long balls and jump balls out of the air.
These knocks don't mean he won't be successful, but maybe explain why he fell to where he did.