One thing to remember about the mobile chips: They rely on thermal throttling a lot.
For small tasks they'll move along nicely and happily, but if they're not getting cooled well enough (As is frequently seen in desktop replacement laptops) then they'll slowly saturate their heat sink and then throttle your speed back heavily. They're great for quick, short session number crunching, editing a few photos for example, but they can really bog down and choke if you try to do things like render out long video on them.
Processor Base Frequency 2.9 GHz
Max Turbo Frequency 3.8 GHz
So when it is cool and hasn't been doing much you can expect 3.8GHz, but can probably expect it to dip down closer to the 2.9-3Ghz not too long after if you're doing much. If you keep hammering away at the heavy numbers then it can dip even lower. Adding in beefy graphics cards and the like into a small form factor like a laptop, and you're not in for the best time of your life. Also, watch out for putting the thing in your lap or on laminate table surfaces...
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500