Experimental mipmaps support. If you are not familiar with mipmaps, here's the basic idea. Instead of working on a huge image in its entirety, an application generates a smaller version of the original image and processes it for preview. While you are evaluating the preview, it silently chews the real thing in the background. Again, it's an experimental feature currently not used by GIMP, whether it will prove to be GIMP 2.10 material depends on contributors activity.