If you've read my posts around the board over the years, I'm very big on a metric called Agility Score, which was developed by Shawn Siegele over at RotoViz. Essentially, you take the 3-Cone and Short Shuttle and add them together (very complicated), and it gives a single number measure of agility from a RB. For little RBs expected to be COP guys or third down backs primarily, this doesn't really matter that much. Sub-200lb guys better be able to move and cut or they won't make it in the NFL. However, when you get big dudes (220+) who can change directions on a dime, that correlates pretty highly with NFL success.
I posted some of this way back on
page 5 of the Ty Montgomery thread last year, but look at Agility Scores for drafted running backs weighing 220+ from 2013-2015;
*** Note - some of the elites at the position didn't complete full workouts, so we have no data on them - Gurley, Zeke, etc.
*** Eddie Lacy (231 lbs) ran a 7.33 3-Cone which would've ranked slower than all but Jeremy Hill on the list below. Lacy didn't run the short shuttle, so we don't have an agility score
*** A single asterisk (*) denotes pro-day numbers instead of combine numbers
2013 - Christine Michael (220 lbs) - 6.69 3-Cone; 4.02 Short Shuttle -- 10.71 Agility Score
2014 - Tyler Gaffney (220 lbs) - 6.78 3-Cone; 4.18 Short Shuttle -- 10.96 Agility Score
2013 - Le'Veon Bell (230 lbs) - 6.75 3-Cone; 4.24 Short Shuttle -- 10.99 Agility Score
2015 - Malcolm Brown (224lbs) - 6.86 3-Cone; 4.15 Short Shuttle -- 11.01 Agility Score
2015 - David Johnson (224lbs) - 6.82 3-Cone; 4.27 Short Shuttle -- 11.09 Agility Score
2014 - Lorenzo Taliaferro (229 lbs) - 6.88 3-Cone; 4.22 Short Shuttle -- 11.10 Agility Score
2015 - Jay Ajayi (221lbs) - 7.10 3-Cone; 4.10 Short Shuttle -- 11.20 Agility Score
2015 - Zach Zenner (223lbs) - 7.08 3-Cone; 4.14 Short Shuttle -- 11.22 Agility Score
2015 - Buck Allen (221lbs) - 6.96 3-Cone; 4.28 Short Shuttle -- 11.24 Agility Score
*2013 - Mike James (223 lbs) - 6.93 3-Cone; 4.34 Short Shuttle -- 11.27 Agility Score
*2015 - Matt Jones (231 lbs) - 6.84 3-Cone; 4.44 Short Shuttle -- 11.28 Agility Score
2014 - Andre Williams (230 lbs) - 7.27 3-Cone; 4.06 Short Shuttle -- 11.33 Agility Score
*2013 - Spencer Ware (228 lbs) - 7.07 3-Cone; 4.27 Short Shuttle - 11.34 Agility Score
2013 - Knile Davis (227 lbs) - 6.96 3-Cone; 4.38 Short Shuttle -- 11.34 Agility Score
2015 - TJ Yeldon (226lbs) - 7.19 3-Cone; 4.22 Short Shuttle -- 11.41 Agility Score
*2015 - David Cobb (229 lbs) - 7.01 3-Cone; 4.55 Short Shuttle -- 11.56 Agility Score
2015 - Karlos Williams (230lbs) - 7.16 3-Cone; 4.46 Short Shuttle -- 11.62 Agility Score
2014 - Alfred Blue (223 lbs) - 7.15 3-Cone; 4.50 Short Shuttle -- 11.65 Agility Score
*2014 - Terrance West (225 lbs) - 7.28 3-Cone; 4.44 Short Shuttle -- 11.72 Agility Score
*2014 - Jeremy Hill (233lbs) - 7.64 3-Cone; 4.59 Short Shuttle -- 12.23 Agility Score
Carson's Agility Score was 11.81, worse than everyone but Jeremy Hill on this list (thus the Yeti comments). And he did this at 218lbs, below the 220+ threshold set for this list. His 3-Cone of 7.53 would've ranked slower than everyone but Jeremy Hill (even Lacy bested that by 2 tenths of a second). His Short Shuttle of 4.28 was middle of the pack, but still was beat by 11 of the 20 we have here. Carson is an explosive athlete given his very good vertical and broad jumps, and he's not terribly slow with a 4.58 40 (though that's not blazing).
Mixon didn't have a best-in-class agility score for this list either (11.37) but he did so at 228 lbs and still far outpaced Carson.
When the metrics align with the film, I feel pretty confident in saying that I'll be surprised in the long run if Carson ends up a multiyear RB2+. He may very well hold the job this year, and even next year, but guys like him tend to eventually get replaced.
ETA - obviously being a good athlete doesn't guarantee NFL success, but being a bad athlete generally leads to an NFL team trying to upgrade.