I don’t think contentScaleFactor will do what you want. The contentScaleFactor lets you work at a different resolution to the native resolution, which for high resolution displays may be a more natural resolution.
But to do so you prepare assets that are 2x or 3x. i.e. instead of addressing all 2560 x 1440 pixels directly you work at a 1280 by 720 resolution. But your 2x assets have 4x as many pixels so when filling the screen with a single texture it is still 2560 x 1440. Even if you scale up a lower resolution texture to fill the screen the fill rate is the same.
The other bottleneck in rendering is vertex uploading. But that will not normally change with the resolution, if you have the same number of Image, Quad or Mesh objects.