Pace isn't that important for #10, as long as it's not a debilitating sloth-like issue - which it isn't in James' case. This is a highly specialized position we're taking about in football where skill and tactical intelligence is more important than blistering athleticism, since it's not the 100m at the Olympics. Infact, you could probably count the number of top-notch speedmerchant #10s over the last decade on one hand, as opposed to an abundance of medium paced ones.
José has fielded the likes of Deco, Lampard, Sneijder, Özil, and Oscar at that position in the past, and while they were very different type of players, none of them were rapid in the mold of a Kaká. James would be a good fit in a counter-attacking setup (which is usually the attacking tactic n a Mourinho team), and given his uncertain situation at Madrid - might be one of the best #10s (outside of the borderline unrealistic ones) we could hope to sign in the current market.