I'm not that familiar with him but if you google him the lack of speed is a pretty common point so it's either from his 40 time at the combine, or actual production or both.
I won't write him off his 40 time is faster than Boldin's was for example and look how good he panned out, you never know. He just seems redundant to me. MHJ doesn't play as fast as we were told he was. Mike is great but he's not a burner. Bourne I like, but again not a burner. We have a bunch of big WR's, Bourne is the smallest at 6'1" but not much speed. Doesn't mean it's impossible to overcome the Rams have had a great passing game and they didn't have burners either. So maybe CMLF will be able to create a good offense