I guess you arent aware that 3D games and 2D UI elements are not powered in the same way. 2D even at QHD is effortless for most GPUs, 3D games are completely different and require a completely different set of calculations and physics.
The limitation on 2D is bandwidth, getting the frames out fast enough, the calculations needed to render the frames are not a problem at all. The GPU has more than enough bandwidth for QHD 2D. How else do you think the GPU manages to handle 4K video recording? In that situation it also has to encode the video as well as render the frames for the screen. Its not problem at all.
The Find 7 801 QHD has the exact same problem. Frame rate dropping,stutter and lag. Coincidence?
This is a hardware issue without a doubt. Again, run the frame rate app in the background on a 1080 801 then the QHD 805 and look at the low frame rates on the G3. Also go and read Anandtechs comments about needing an 805 with QHD to get 801 performance on 1080.Look at Erica Griffins video where she states the 805 G3 is much smoother than the 801 G3 and advises against buying the 801 G3(both the same LG software),finally go and read the comments on the Cat 6 thread where they say their 805 G3s are running smoothly with no lag and as fast as an M8. The 801 is capable of 2k,just not very smoothly.