There have been a couple threads about lag on the iPhone 6 Plus and about whether or not lag exists. Let me set the record straight, yes, lag exists.
This simply due to the resolution the GPU is rendering at. On the 6 Plus, the quad-core GPU in the A8 is rendering everything at 2208 X 1242 and on the 6 it is rendering everything at 1334 X 750. Obviously more pixels = more power required.
Also, the 6 Plus is scaling all the content from 2208 X 1242 to 1920 X 1080. However, this sort of scaling is extremely light on the GPU and does not have a major affect on performance despite what many people on here say.
So back to lag. It exists. But there is a "fix" for it. When you go into Settings > Display & Brightness > View, you select between either "Standard" or "Zoomed". Standard is rendering everything and scaling all the content from 2208 X 1242 to 1920 X 1080. Zoomed is rendering at 1334 X 750 and scaling to 1920 X 1080. And again, this scaling is super light on the GPU and nothing to be concerned about.
So with "Zoomed", you get iPhone 6 performance on the iPhone 6 Plus. However, there are a few downsides. PPI equivalent will be less than 326, not sure the exact number, but if the iPhone 6 is 326 and the 6 Plus takes that and scales it up to fit the display, the PPI equivalent must be less than 326. Also, you lose special landscape apps, landscape keyboard, and rotation of the homescreen. You are basically taking the entire iOS UI of the iPhone 6 and scaling it to the 6 Plus. So you lose some features, but you gain performance. That is why "fix" is in quotes, because it is not a real fix.
There is definitely a trade-off between the two. Added landscape features and resolution vs. better performance system wide. Just know, there is a way around lag if you really want it.
This simply due to the resolution the GPU is rendering at. On the 6 Plus, the quad-core GPU in the A8 is rendering everything at 2208 X 1242 and on the 6 it is rendering everything at 1334 X 750. Obviously more pixels = more power required.
Also, the 6 Plus is scaling all the content from 2208 X 1242 to 1920 X 1080. However, this sort of scaling is extremely light on the GPU and does not have a major affect on performance despite what many people on here say.
So back to lag. It exists. But there is a "fix" for it. When you go into Settings > Display & Brightness > View, you select between either "Standard" or "Zoomed". Standard is rendering everything and scaling all the content from 2208 X 1242 to 1920 X 1080. Zoomed is rendering at 1334 X 750 and scaling to 1920 X 1080. And again, this scaling is super light on the GPU and nothing to be concerned about.
So with "Zoomed", you get iPhone 6 performance on the iPhone 6 Plus. However, there are a few downsides. PPI equivalent will be less than 326, not sure the exact number, but if the iPhone 6 is 326 and the 6 Plus takes that and scales it up to fit the display, the PPI equivalent must be less than 326. Also, you lose special landscape apps, landscape keyboard, and rotation of the homescreen. You are basically taking the entire iOS UI of the iPhone 6 and scaling it to the 6 Plus. So you lose some features, but you gain performance. That is why "fix" is in quotes, because it is not a real fix.
There is definitely a trade-off between the two. Added landscape features and resolution vs. better performance system wide. Just know, there is a way around lag if you really want it.