To understand the problem fully, you have to make a serious distinction between first and second generation Airs.
I have both, and the first generation becomes completely unuseable rather quickly, external display or not, when you push it even mildly. (Youtube (non-HD) alone will do it.)
The second generation is much more resistant, but if the room gets hot, with my 24" external monitor connected, I do see the kernel_task slowdown at times. (E.g., when a Time Machine backup kicks in, with 10-12 apps going, mostly in the background, and I'm doing something mildly computation-intensive in the foreground.)
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To understand the problem fully, you have to make a serious distinction between first and second generation Airs.
I have both, and the first generation becomes completely unuseable rather quickly, external display or not, when you push it even mildly. (Youtube (non-HD) alone will do it.)
The second generation is much more resistant, but if the room gets hot, with my 24" external monitor connected, I do see the kernel_task slowdown at times. (E.g., when a Time Machine backup kicks in, with 10-12 apps going, mostly in the background, and I'm doing something mildly computation-intensive in the foreground.)