That's fair. It's not like Gronkowski or Ertz are the speediest guys. Kittle's field speed doesn't look like 4.52.
As for seam busting, a common topic when it comes to TEs, that's predominantly a function of zone route running, body control, competitiveness at the ball, and no shortage of fearlessness. Whether an ILB is man-chasing, or dropping to the under zone, the receiver has to find space under or away from the high safety where the QB expects to find him.
Now, being able to outrun that ILB when the FS abandons the middle of the field is a valuable trait, but the opportunity is pretty rare. Even if you have a speedy TE, and the FS is going to move to low-high zone on the perimeter, you're going to see the SS move into middle position or cover up with a hybrid ILB matching speed with speed.