Lightroom doesn't use the GPU to process photos, so the bottleneck is in the CPU. The CPU has to render photos for over 8,000,000 pixels on your screen, compared to the standard HD screen which has about 2 million pixels, so of course Lightroom is going to be slower rendering the photos.
I think you are right, but I made some testing lowering the resolution to Full HD and still the lag is the same.
I’d guess video-driver issues, or more likely I/O or memory or video-memory bandwidth problems since you only have an i5 and a massive screen.
The way to tell if it’s CPU is to run Task Manager and look at the CPU usage of the LR process. You can also monitor the video card with something like CPU-Z from cpuid.com or other GPU monitoring programs.
Keep in mind that LR is not video accelerated so all the display pixels have to be pushed to the video card from the CPU.
If you resize LR to only use 1/4th of your screen does it speed up? This should be the same number of pixels as a non-ultra-sized monitor.
With 1:4 picture size, the effect are almost fluid. So again this lead again to the CPU.
Some testing later...
Editing a picture in Full HD resolution in 1:4 zoom isn't smoother than editing a picture in UHD resolution in 1:4 zoom.
Still haven't figured out what going on...
Unless the CPU is maxed out, it's not the CPU, and more likely something related to the amount of data being computed and moved to the video display memory. You can get to task-manager by right-clicking on the task bar and choosing it from the menu that pops up.
And 1:4 zoom is not the same thing as LR's interface resized to take up only 1/4th of the screen.