Here's a Kittle tape with some successful seam routes, with little of it it having to do with speed.
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I've noted previously that Kittle looks a lot bigger and somewhat slower than a 6' 4", 247 lb. player running 4.52 which was the player Kittle was at the Combine. His upper body approaches that of an OT.
I painfully draw your attention to the play at 5:41 well illustrated from the end zone view. This was about play design and route running, not speed.
Kittle attacks Savage with the flag move, gets him turned around, and breaks to space. With Kittle in full stride, Alexander closes 10 yards on Kittle's last 25 yards. There's no notable speed in any of this.