One word - cooling. Making very good and quiet cooling for thick notes is not the problem, for very thin one it is usually nightmare that still results in CPU throttling under heavy load. Chart above show the issue with fanless design (and most fanless tablets throttle CPU).
I can confirm this. I'm using a 10" tab (not a thin one) that can last 50% CPU overclocking and 100% GPU overclocking at same time without getting noticebly warmer. Awsome.
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