This is most likely down to the MacBook having flash-storage fitted, as opposed to the slower traditional hard drive in the iMac. The desktop machine did fare slightly better in our Geekbench 3 test, where it narrowly beat the MacBook, and both machines scored exactly the same in Cinebench R11.5.
Essentially, at its core, the entry-level iMac is a MacBook Air with a bigger screen, bigger (but slower) storage, and a little more RAM. This means that you could turn either MacBook Air into an iMac just by plugging in a screen, mouse and keyboard, with the option of external storage. Admittedly all of this costs money, but you would end up with a desktop and a laptop for not much more than the price of the iMac itself.
This only holds true for the entry-level model though, as once you step up to more powerful iMacs the gap in power begins to show. There is still the factor of the hard drives in the iMacs (which Apple could eventually change by offering a Fusion drive as standard), but the graphical prowess and CPU speeds are certainly a big improvement, with the 2.7GHz 21-inch iMac scoring a respectable 179 in Speedmark 9 tests, easily ahead of the Airs and the slow entry-level iMac.
iMac vs MacBook Air: Storage options
The tradeoff for the performance boost of the flash-storage in the MacBook Airs is actual storage space, as flash-storage remains expensive for higher capacity units. If you want to keep all your music, home videos, or music collection on your computer, then the Airs will fill up a lot faster than the capacious iMacs. Of course you can use an external USB hard drives, and they are quite affordable these days, with a quick look on Amazon showing that you can buy 1TB drives for less than £50.
Another option would be to buy the 256GB version of the 11-inch MacBook Air, which costs the same as the entry-level iMac (£899). This still leaves it with half the storage, but would be more than enough for most people, especially now that cloud storage is becoming so ubiquitous and cheap.
Conversely you could upgrade the internal drive in the iMac to a 1TB Fusion drive (a mixture of flash-storage and HD) or the same 256GB flash-storage as the MacBook Air, either of which add £200 to the base price of the machine, taking it to £1099. This will definitely improve performance but by that point you're actually getting into the higher specced iMac price range, with a 21-inch model featuring a faster 2.7GHz Intel core i5, Iris Pro Graphics, 8GB of RAM, and a 1TB hard drive, costing £1049.
Sign up for MIS Asia eNewsletters.