https://forums.macrumors.com/threads/1775525/
Make sure to read the whole thing, it has some links in there going step by step showing you how the 6 Plus draws the image versus the iPads and iPhones.
The iPad Air might be pushing more pixels, but thats not the issue. To be able to put more info on the display, Apple increased the point count for the display and the iPhone 6 Plus requests 3x Retina imagine versus 2x for other Retina displays. This is still not where the issue is yet.
Apple gave the iPhone 6 Plus an official point count of 414x736. Now with the other Retina displays, this would be half the resolution. For instance, the point count on the 3.5" Retina iPhones is 320x480, which is the same resolution they used for the non-Retina iPhones (original, 3G, 3GS). They did this to make it easier to just scale the apps up. The resolution of the iPhone 4 & 4s is 640x960 (2x of 320x480).
Now see with iPhone 6 4.7", the point count is 375x667 which when rendered to Retina x2 is 750x1334, or the same resolution of the phone. With the Plus however, the point count is 414x736. Retina 2x would render at 828x1472, not doable. So lets up it to 3x. 1242x2208. So this takes care of the size of framework needed, but now this brings a new issue to the plate. An image of the OS rendered at a size that is too big for the display. Whats it going to do? Resize the image down to 1080x1920. That means to display the image on the display, iOS now has to go back to the GPU a second time in the same amount of time. Because it does have to go back twice, the GPU in the phone would have to be a heck of a lot better then the one found in the iPad Air. The iPad Air may have 500,000 more pixels then the iPhone 6 Plus, however the plus has to reach back into the GPU a second time to resize the image to fit the display.
Now, you may be wondering why Apple did this. A lot of people (myself included) believe Apple may have made another last minute change to the display before shipping. The original display, which was more then likely 1242x2208, may have cost too much to make or they were having problems manufacturing them. So they decided to replace them with something more readily available, 1080p displays.
If this is the case, then doubling the RAM in the Plus to 2GB would increase performance since it would give the GPU more room to write to VRAM (VRAM is taken from the main RAM since its all in the SoC). Some have also said that there will be two revisions of the A8, one found in the iPhone 6 and soon to be iPad mini 3rd Gen and one found in the iPhone 6 Plus and 2nd Gen iPad Air. Why? Well the iPad Air A7 is actually different then the reset of the A7s. While it may only be 100MHz faster, the way things are arranged inside the SoC is more like the A5X and A6X, however it only has dual core graphics instead of quad core found in the A5X and A6X. This makes the same GPU more efficient and the processor 100Hz faster. In fact if you look, the A7 in the iPad Air even has a heat spreader like the A5/A6X. The A8 would probably be the same way.
Now, I'm not saying the iPhone 6 Plus will be a slow clunky phone. (Heck, I ordered one for my self since I have been wanting a bigger iPhone for better video viewing). I'm saying that more then likely it will have a slightly revised and more GPU-centric SoC then the A8 in the iPhone 6.
PS: For those who might ask why Apple couldn't have just changed the point count for 1080p at Retina 3x scaling, its because the point count would then be 360x640, meaning the phone would have a smaller point count then the iPhone 6 (which is 375x667). Using Retina 2x imaging wouldn't have come out as clear.
