See Adobe’s Eric Chan’s post GPU notes for Lightroom CC (2015) for a behind-the-scenes explanation of how Adobe are adding GPU support:

Lr can now use graphics processors (GPUs) to accelerate interactive image editing in Develop. A big reason that we started here is the recent development and increased availability of high-res displays, such as 4K and 5K monitors. To give you some numbers: a standard HD screen is 2 megapixels (MP), a MacBook Retina Pro 15″ is 5 MP, a 4K display is 8 MP, and a 5K display is a whopping 15 MP. This means on a 4K display we need to render and display 4 times as many pixels as on a standard HD display. Using the GPU can provide a significant speedup (10x or more) on high-res displays. The bigger the screen, the bigger the win….

let’s be clear: This is the beginning of the GPU story for Lightroom, not the end. The vision here is to expand our use of the GPU and other technologies over time to improve performance. I know that many photographers have been asking us for improved performance for a long time, and we’re trying to respond to that. Please understand this is a big step in that direction, but it’s just the first step. The rest of it will take some time.

Also see Eric’s comments here and though I’ve already linked to it, Adobe’s GPU Troubleshooting & FAQ is also important.