Back, one more time (at least) to the Bill James Handbook series.
It used to be a given: Joe Mauer was never particularly fast, but he was a good baserunner.
He's older now, older and slower. And his baserunning is no longer a positive.
In 2013 -- Mauer's last season as a catcher, and his last as the Hall-of-Fame caliber hitter he had always been to that point -- Mauer was, by Baseball Info Systems' reckoning, a +15 as a baserunner. He was on first base for 31 singles and went to third on 12 of those. He went second to home on 10 of 18 opportunities. He scored from first three times on 11 doubles. And, most tellingly, he was never thrown out taking an extra base and never doubled off a base.
Mauer in 2016 was considerably less adept. He went first to third only five times in 23 opportunities. He was thrown out twice and doubled off once. All told. BIS has him at -2.
There are, to be sure, worse baserunners on the Twins than Mauer -- Trevor Plouffe, for example, was -14 last season, and Miguel Sano -4. And there's more involved than just speed: While Byron Buxton was a +28, Danny Santana was merely +1.
For his career, Mauer is still an impressive +93 -- astoundingly good for a catcher/first baseman. But it's slipping. Which should be no surprise. He'll turn 34 early next season, and the much-touted water system at Target Field does not include a fountain of youth.