@Intrinsic: If you enable DSR, you are making the GPU render the game at said resolution, not upscaling the image to that resolution.
The Teach Report did a good article about it : http://techreport.com/review/27102/maxwell-dynamic-super-resolution-explored
Enabling DSR at 4K and downscaling the image to 1080p is actually more demanding than just rendering the image in 4K.
Please excuse my bad English.
Currently gaming on a PC with an i5-4670k@stock (for now), 16Gb RAM 1600 MHz and a GTX 1070
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