Rico does look fairly slow in those clips but his 40 time (what I found anyways) was 4.75 which isn't exactly terrible for a TE. I did a quick check on 40 times for some of the top TEs and he is on the slower end but not too far off. I'm not saying he is going to become one of the better TEs out there but his speed shouldn't limit him that much.
Delanie Walker: 4.49, 6' 2", 248 lbs
Greg Olson: 4.51, 6' 5", 255 lbs
Coby Fleener: 4.51, 6' 6", 251 lbs
Zach Miller: 4.53, 6' 5", 243 lbs
Travis Kelce: 4.63, 6' 5", 260 lbs
Jason Witten: 4.65, 6' 6", 263 lbs
Gary Barnidge: 4.65, 6' 6", 250 lbs
Tyler Eifert: 4.68, 6' 6", 250 lbs
Rob Gronkowski: 4.68, 6' 6", 265 lbs
Julius Thomas: 4.68, 6' 5", 256 lbs
Martellus Bennett: 4.68, 6' 6", 275 lbs
Jordan Reed: 4.72, 6' 2", 246 lbs
Rico Gathers: 4.75, 6' 6", 281 lbs
Zach Ertz: 4.76, 6' 5", 250 lbs