He's been nursing a hamstring which hasn't allowed him to really do anything as far working out. He only started running a week ago.
There are no work ethic issues with Lacy. The knocks on Lacy are durability and ball security... the latter of which has been fixed under Coach Burns, and hasn't been an issue since early in his career.
The speed is fine, especially for a 230 pound back that can break 4.5 when completely healthy. The most productive rookie running back in last year's draft ran 4.63 (Alfred Morris). Vick Ballard ran 4.59. Hell, Arian Foster ran 4.69.
40 times are never more overrated than they are with running backs. It's so important to judge the tape with these guys.