He is no burner, but he is not slow.
No doubt. He plays a lot faster than his 40 says he does, IMHO. He was 229 at the combine. I think he could play at 220-222 and gain some quickness, too.
So, we've read his weaknesses. From the same webpage...
STRENGTHS
Powerful, compact runner with a low center of gravity. Efficient and decisive. Makes defenders miss with slight movement and hand usage. Won't succumb to arm tackles. Gained 54 percent of his yardage after contact from 2013-2014. Patient runner who gets the most from his runs. Has good speed in open field for a back his size. Balance is his friend. Smooth change-of-direction with little wasted motion. Gets low and finishes with leg drive. When he smells the end zone, he wants to eat.
http://www.nfl.com/draft/2015/profiles/david-cobb?id=2552376
Needless to say, I disagree with Gaede that he lacks toughness. If Cobb is anything, he is a tough runner. He played behind an ultra-crappy OL and averaged 2.3% more 5+ yard carries than his team's average. Here are those numbers for some other backs...
Jay Ajayi: -1.1%
Mel Gordon: 0.5%
Jeremy Langford: -0.2%
Buck Allen: 0.4%
Tevin Coleman: 1.7%
Duke Johnson: 1.3%
Mike Davis: -2.1%
Todd Gurley: 1.3%
TJ Yeldon: -0.9%
http://www.footballstudyhall.com/2015/2/2/7959095/college-football-running-back-stats-melvin-gordon
Is Cobb gonna rip off a bunch of 50 yarders and be a scoring threat on every touch? No, but he will get you positive yards on every carry, average 4.5 per on 20 runs a game and beat the crap out of the defense in the process. I think that's as close to Murray as you're gonna get.