Exactly, what Xandros said. They're low power parts, full voltage parts usually either end with MQ or HQ or some such derivative at the end of their name - and the voltage and clockspeed delta between them makes a big difference in performance. Most processors you will find in Ultrabooks, slim and thins, and tablets are going to be U series parts, with low TDP packages for better thermals and batterylife, but far worse performance for specific or such oriented applications.
And also, the branding between them differs/varies widely. I'll give an example here. The i7 parts you see in all the ultrabooks, tablets, and the surface lines as well - with a premium charge on them - are basically just higher clocked, higher binned dual core parts with hyper threading enabled. Outside of higher turbos, and better clocking and binning, IGP, theres virtually no difference between one of these, and an i5 part. These are all ULV or low powered processors ofcourse. However, on a full powered mobile part, which would have a package power of between 35-55 watts, I forget, the i5 will be a dualcore, hyperthreaded part, and an i7 will be a quadcore, hyperthreaded part, with similar clockspeeds et al. A difference of two extra cores, and four more threads.
In real life scenarios, this is how it plays out. You open a large CAD file, or a large Rhino/3DSMax/Sketchup model, or set up a high definition V-Ray/Lumion/et cetera render, and your ULV i7 and i5 machines are going to come crashing down to their knees, in a slow crawl, whilst the full voltage mobile parts are going to take a little while longer before they start falling apart in the same vein, hopefully shaving you a few minutes off those important renders, or modeling or drafting a little faster for a while more. Your mileage may vary across different disciplines, ofcourse.
On a performance grouping, its going to be the mobile i5, the ULV i7, and the ULV i5, in that order, for performance in most cases, grouped in the same class, and the the mobile i7, in a separate performance delta, slightly higher than other three. A good GPU is also a good idea, usually though, just saying.
Hope this was kind of exhaustive, for said purposes.
-- W
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