There's a lot of incorrect/misleading information in this thread.
First, if you write your *entire* app in OpenGL or Metal, using the correct frame buffer size for the iPhone 6 and iPhone 6 Plus, you'll get very close rendering speeds/fps. Both phones have the same GPU. The iPhone 6 Plus won't magically become faster than the 6.
Second, if your app isn't a gaming app, your use of OpenGL or Metal is either very little, or none (some exceptions are Facebook Paper and Flipboard). The lag you're seeing is from UIKit, which is what *all* native iOS apps use (and by native, I mean you're not using a 3rd party dev platform to mimic UIKit in OpenGL). The lag is a result of the GPU having to downscale. Again, it's the *same* GPU in both devices.
Third, if there's any software tweaks to squeeze more performance out of the 6 Plus, those tweaks will also apply to the 6. Same CPU, GPU, RAM, and OS.
This is how you write software for iOS - Apple gives you an SDK with a bunch of APIs. How those APIs work aren't public. You can't make them 'faster' or 'better.' That's Apple's job.