Different MBAs have different CPUs -- I remember having the option to upgrade when I bought mine, and the base 13" is beefier than the upgraded 11". e: Ah, but

Different MBAs have different CPUs -- I remember having the option to upgrade when I bought mine, and the base 13" is beefier than the upgraded 11". e: Ah, but it actually runs fine on my 2010 unupgraded 11" Air. I'd guess a higher resolution screen would hurt performance, though.