When I buy a new machine, I like to test its power by pushing it to the limits until I discover where the cut off point is before it starts to lag and beach ball etc. So how far have you pushed your 2011 Macbook Airs before you started to notice lag or before a beach ball got in the way? What apps did you run to push it? What was the memory and CPU usage like? Please try and post the MBA details (11"/13" i5/i7) as well.