I wouldn't say it's his lack of pace, that's not really a major attribute needed for a player of his style. You don't see many quick centre midfielders around, players like Xavi, Alonso, Pirlo and Scholes are as slow as they come.
I just think the likes of Xavi and Iniesta are more complete footballers and at the moment, more experienced players playing in a system that suits both of them perfectly.
But that's my point. He looks slow compared to Xavi.
Yes, their experience probably does help them but they also seem to have the edge over him in terms of ability.
Wenger has started playing him off the front man as he's had to compensate for his lack of physical strength/speed.
If the space is there for him, or if the play is so tight that speed can't be exploited he's fantastic. If the game develops to a point where a lot of running is needed he's not quite there.
Although he clearly has time to work on things that minimise these problems for him.