There are a TON of quality safeties that run a laser timed 4.60-4.70, but are much better athletes than that. I've coached at the collegiate level and I can tell you this. You would be SHOCKED at how many kids aren't track athletes and have no idea how to "get out of the blocks" so to speak in the 40 yard dash... A lot of kids are much faster than their time indicates, but they are terribly slow starters (because of technique), but fluid, fast twitch athletes that read and react well... Two quick stories as examples.
1. We had an All Canadian running back who is now playing in the CFL and was at one time, on a NFL practice squad. He was extremely highly recruited here in Canada and had some mid level interest in the U.S. among D1 schools. The reason he didn't have more interest was his 40 time, which was 4.81 laser-timed at a combine coming out of high school. The first time I saw him test his 40 time it was evident he was a brutal starter. He was "overtrained" in his Olympic lifts and although he was an impressive looking ball of muscle, his hip flexers and his core muscles were not flexible and it made him a rigid, slow starter. Not that he was a rigid athlete, but he needed to loosen up that region of his body. We got him hooked up with a personal trainer in Toronto, the winter before he reported to our training camp. That guy worked a lot on his flexibility and his explosiveness and basically made him put down the heavy weights. He kept his strength up, but also limbered him up and worked on his running technique and the bio mechanics of running. He can to our camp and on testing day he ran 4.73, nearly .1 of second faster. We redshirted him as a freshman and our strength coach worked with him tirelessly to continue to loosen up his core, his hips and his gluts. Our strength coach worked at one time with the Canadian Olympic Gymnastic team and had keen understanding of how to increase flexibility and durability in athletes. He was great improving functional strength By time he tested his 40 in his second season with us, he was 5'11, 194lbs, he had really good lifts numbers and he was infinitely more flexible than he had been. That second testing day he ran 4.66 and every year he improved. When he went to the CFL combine, 3 years later, he was incredibly flexible, over 200lbs and ran 4.58... There is so much more to it than simply looking at a guy's time at 18 years old and saying "he's slow".
2. We signed a 6'4, 205lb wide receiver, who had played 1 year of football. We were the only team to offer him a scholarship. He was a very good basketball player, but played as a power forward his who life and stopped growing around age 16 and eventually he knew that he wasn't going anywhere with basketball because he didn't shoot well enough to be a guard, but would be dwarfed at the collegiate level as a forward. His senior year of high school he switched to football and he was physical, fluid athlete. All the years of playing basketball made him a much more limber, flexible athlete and he wasn't bulky and muscular, but rather, he was a lean, toned athlete, with strong, naturally developed legs and he was use to sprinting. He actually ran his first ever laser timed 40 at our summer camp and clocked a 4.47. He's had a great beginning to his career at the university and I expect him to be a really good player down the road. He hasn't gotten much faster, but instead, he's focused on getting stronger. He still runs sub 4.5, but he's now about 215lbs and solid.
Definitely don't live-and-die on speed of 18 year olds... Consider this. Would you want Zibby or Bruton at safety, or Max Redfield? Because I guarantee you that on paper and at a combine, Max blows those two guys away in the athleticism department, but those guys are fast enough and smart enough, to not only be impact players in college, but also went on to play a little bit of ball in the NFL.
If Devin Studstill runs 4.76, but has great instincts, a nose for the ball, the desire to get off the hash, and the work ethic to improve, he's be sub 4.7 at some point and likely an upgrade from what we have.