This was the word on him coming outta college...
"Doesn't possess the kind of acceleration that gets him up to top speed quickly.
Agility: Even though he shows some agility, Gronkowski won't make a lot of people miss in the open field. May not have the best vertical leap, but he's an intelligent player who can high-point the ball." "Post-Draft Outlook: BEST PICK: Tight end Rob Gronkowski: The soft-handed product out of Arizona convinced New England doctors that he was healthy enough to warrant a second-round pick. Gronkowski fills a huge need and could be expected to contribute from day one.
A closer look at the Patriots' picks: Round 2/42 -- Rob Gronkowski, TE, 6-6, 265, Arizona, After sitting out 2009 with a back injury, Gronkowski left college early as one of the top tight end prospects. He has soft hands and can block, filling a huge hole in New England. - by The Sports Xchange"
Nobody was talking about him being some fast and athletic TE.
That doesn't mean he was slow but compared to Kelce he's not that. Gronk's strength has always been his size and his hands. Step it up.