Is iPhone 6s plus actually slower than iPhone 6s with the same cpu, gpu and ram?
No. If it has the same hardware specifications, it should not be any slower.
As the resolution of iPhone 6s Plus is really higher than 6s, it means that it needs more ram and gpu(?) power to process more screen data/image buffer.
No. There's no specific amount of processing power needed. You'll use whatever processing power you have to do whatever work you need to do.
But even if this was true, obviously Apple would make sure the iPhone 6s Plus has whatever processing power it needs.
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Questions about electronic devices, media players, cell phones or smart phones are off-topic, except insofar as they interface with your computer, See On Topic.
Try asking on http://apple.stackexchange.com/
If it has the same CPU, GPU, and the same amount of memory then it is literally not possible for one to be faster then the other. – Ramhound – 2015-11-20T18:42:41.973
@Ramhound if hardware is same except the screen resolution, won't it use more power, ram to fill the extra screen space? – frankish – 2015-11-20T19:36:17.710
@DavidPostill Sorry, I thought that this is the right place. Actually this is not directly related to Apple, it could be another brand too. Which stackexchange site do you recommend? – frankish – 2015-11-20T19:37:18.710
@frankish - There isn't a proper Stackexchange website for this question, it isn't a good question, you don't have an actual problem. It's sole purpose seems to generate a discussion on the topic. Also....I never said the two devices didn't consume different amounts of power. Please read my comment, and only what I actually said, would be the same. The CPU computational power are identical though. – Ramhound – 2015-11-20T19:40:22.973