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Is it possible that what's taxing the device is the fact that the resolution is downscaled?

iOS 8.0 seems slightly unfinished to me, 8.0.1 should be good though.

These seem like possible explanations to me, which of course means the (possible) issue could be fixed with a software update.

As others have pointed out it doesn't make any sense that the more powerful A8 chip couldn't properly handle lower resolution than the A7 in current devices. However it seems logical that the iPhone 6 plus would be the worse performer out of the two because of the higher res.

And everyone remember this is mere speculating!
 
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