The fastest baserunner in the game is Billy Hamilton. Let's get that out of the way as quickly as possible. You know that, and so do we; it's one of those cases where the eye test and data align perfectly.
But who's second, and who's last, and how big of a gap can there be between a runner with elite speed and a slow one? What's a satisfying way to measure that?
• MLB's fastest baserunners by position
We took a first step toward answering those questions in April when we introduced the Statcast™ speed metric Sprint Speed, though at the time, it was only for outfielders on defense, and full leaderboards weren't available. We promised that we would shortly follow up with the same information for all baserunners and public leaderboards, and now we have. The leaderboards are here on Baseball Savant. Enjoy them.

Through Monday, more than 350 players qualified by having at least 10 maximum effort runs tracked. Check out the top 10 leaders for a sense of who the fastest runners in the game right now are:
2017 Baserunning Sprint Speed leaders, feet per second
30.1 feet per second -- Billy Hamilton
29.9 -- Byron Buxton
29.8 -- Bradley Zimmer
29.6 -- Franchy Cordero
29.4 -- Keon Broxton
29.3 -- Dee Gordon / Delino DeShields
29.2 -- Kevin Kiermaier / Manuel Margot / Lorenzo Cain / Jarrod Dyson / Mallex Smith
27.0 -- Major League average
So, how does this all work? As we discussed in April, we're measuring speed in terms of "feet per second in a player's fastest one-second window," because feet and seconds make a lot more sense in the context of baseball than miles and hours. While three feet per second may not sound like much, if you were to maintain that speed for four or five seconds, suddenly you're talking 12-15 feet. It can be the difference between being safe or out or not even trying. While Hamilton averages about 30 feet per second, Olympian Usain Bolt, by comparison, has reached up to 37 feet per second in his first 40 meters.
Of course, there are so many plays where a runner is merely jogging to first after having popped up, or trots home easily from third base when a teammate doubles, and those non-competitive plays don't exactly tell us anything about speed, so we had to find a way to exclude those plays and include only plays where maximum effort could be expected.
To account for that, we took all batted balls (excluding over-the-fence home runs), and looked at plays where a runner or hitter attempted to advance two or more bases (excluding runners who started on second base and the batted ball was an extra-base hit, as they can often jog home). Of the remaining runs, we'll sort them from slowest to fastest, and take the average of the fastest half. If that sounds complicated, it needs to be, but the results are extremely satisfying.

As you can see, the range from the top to the bottom runs approximately 6 feet per second, which can add up to an enormous difference in a span of several seconds. The next names at the top include Trea Turner and Rajai Davis. At the top, 12 of the fastest 14 play center field; at the bottom, 24 of the slowest 25 are catchers, first basemen or designated hitters. So far, so good; that's exactly what a list like this ought to show.

As you'd expect, there's an extremely strong correlation in year-to-year speed, because it's a skill, much moreso than it is an outcome. But what's really interesting here is to look at the three biggest decreasers, because they all have the exact same story: leg injuries. 
While most players hold steady year to year, veteran infielders Josh Donaldson, Luis Valbuena and Adrian Beltre tied for the biggest speed decline, dropping more than a full foot per second, and each have some pretty clear health issues that correlate to foot speed. Valbuena's 2016 ended with a hamstring injury that required surgery, and then he re-injured it again this spring, missing all of April. Donaldson has been plagued by a calf injury all year, playing in only 28 of Toronto's first 58 games, and Beltre didn't make his 2017 season debut until May 29 thanks to a calf injury.
If you're wondering if your favorite player is playing through pain, now we have a better way of investigating. We can do the same for age, too; Dyson, for example, turns 33 in August, and lost half a foot per second from 2015-16, and again from 2016-17.

It's far easier, by the way, to lose speed than to gain it. While 12 players lost at least one foot per second this year, and 71 lost at least a half-foot per second, only three gained a foot per second, and only 22 gained at least a half-foot per second. Among the gainers are Ryan Zimmerman (+0.6 ft/sec), who has attributed health to his great start, and Scooter Gennett (+1.0), who missed time last year due to an oblique injury. The biggest gainer, however, may be a surprise: Aaron Judge (+1.8 ft/sec), who appears to have found yet another leaderboard to conquer.

But back to our leaderboard for a second, because you're surely wondering who in the world "Franchy Cordero" is -- he's a San Diego rookie outfielder off to an impressive start -- and he serves as a valuable way to get into something important here: sample size. You would never look at a hitter who had a single in two at-bats and say he was truly a ".500 hitter," but you don't need to see Albertin Chapman top 100 mph more than once or twice to know that his velocity is real. Different metrics require different amounts of time to be useful; the ones that are purely about skill, such as arm strength or spin rate, tend to take less time than outcomes, such as ERA or OPS.
We're starting to see that foot speed follows the same path. Just look how high up Cleveland's Zimmer appears, or take, for example, Cardinals rookie Magneuris Sierra, who didn't qualify for our leaderboards since he spent just eight days in the Majors earlier this year. He arrived with quite the reputation, and in his brief stint in the bigs, he showed why.

"Sierra is extremely athletic with plus speed that he uses on both sides of the ball," noted MLBPipeline's 2016 scouting report. "It's awesome to watch," Cardinals teammate William Fowler said after Sierra's third Major League game. "The guy comes up and comes right in and contributes. Speed never slumps."
Sierra had six tracked runs that qualified this year, each one falling between 29.0 and 30.8 ft/sec, for an average of 29.9. Remember, 27 is average, and only a half-dozen runners have had even one sprint of 30.8 this year. In just a handful of games, Sierra showed skill that would have put him in the top three if he'd qualified, which he may well do if he returns later this year.
Speed matters. It matters on defense, and on the bases. It peaks early, and it doesn't take long to see if it's real. We've never really been able to measure that in a meaningful way, until now.
The leaderboards are public, and are sortable by team and position. Go have fun -- and tell us what you find.