Swampboy nails it perfectly, completely agree.
And CaCO - there isn't a single example in our local world of a kid who was slow as a pre-teen who became fast because of puberty. Michael27 is right on - my 2018 happens to run track (100m & 200m) & play baseball, and the kids who were fast when he was 8 years old are still the fast ones, period.
Kids do get stronger, more athletic, taller, wider, etc. - but while they may train to get a better 60 time, the true leadoff hitter "speed kills" kids are the same ones who where that way when they were playing AAA Little League. Doesn't mean they don't become better players as puberty hits, but speed is one element that can be determined very early. Have seen multiple fast kids get SLOWER - puberty, getting plumper & lack of work ethic can tend to make them lose their skills. Have yet to see a slow kid get fast though.
I've seen a lot of tall kids go from slow to fast after they hit puberty. Matt Jones, who played QB at Arkansas, ran a 4.3 at the NFL combine. He played O-line on his 7th grade team and started Varsity as a 9th grader at WR. My son's JR high football team is where I have seen the big speed jumps from the tall kids. They time the kids before their speed and agility camp. My son went from a 7.2 40 at 11 to a 5.1 40 at 13. Our top WR went from 6.5 to 4.8. Both kids are over 6 ft tall 13 year olds and I expect they will continue to get faster. Those are just a couple of examples.