He's not a long strider, you're right, but you can see that he doesn't separate deep either. The vision that is running through my head, as I'm writing this, is a play against CB Antwan Allen, a smallish corner from Kirk Ferentz' team Iowa. Marshall gets into his route very fast and shows the good hips to fake (the beginning of the route, kind of looks like a 'chop dance' to me because WRs always chop their hands and stutter step to fake their directio), and you could see him start to separate from the corner right around the intermediate area, but then as he goes deep and toward the sidelines, the lack of top line speed allows the corner to catch up with him. On this play in particular, Betts had rolled out to the sidelines, and instead of coming back to Betts, Marshall waived his arm up in the air signifying he's going a bit deeper, because the corner sat in that zone with his eyes fixed on Betts. Betts launched, the corner ran up under Marshall, but the ball was in his hands before he could do anything to him.
I found that to be a good example of what you expect from Marshall. This is why I think he'll be good on skinny posts, slants, corners, crossing routes, and fades, instead of typical double move or straight up fly or vertical routes...because his top line speed is not going to be a source of separation for him. His intermediate area speed, hips, and second gear are going to be what he relies on to get open in man coverage, and he will rely on his hands, focus, and body control to bring the ball in when he's challenged. In the zone, he seems to have a real natural feel for getting between the holes in the zones.
And he played almost the entire year at safety in 2004. I believe he played like 9 games there, and became their leading tackler because he's such a good athlete. He clearly used that experience to work in his favor, and says it gave him fresh perspective on how to hurt defenses from the offensive side of things.
I would pencil the guy in as a tight end except he's got the body and the body language of a receiver. I don't know that he'd be able to gain weight and become an adequate blocker. I think Marques Colston is closer to being a possible TE convert than Marshall, even though his top line speed is better than Marshall's.