GNOME Work Is Underway For Sharper Background Images
Long story short, for where the background/wallpaper image is larger than the desktop resolution, OpenGL is used for downscaling the image. But the existing means of downscaling could lead to blurry images or just not as sharp as possible images. But now with patches pending, the mipmap level is being limited to still downscale with OpenGL but to have the maximum sharpness possible for the display.
More details on this work-in-progress GNOME background improvement via this merge request.
Some of the other work Van Vugt is pursuing are more efficiency improvements for Clutter, avoiding a CPU bottleneck on the lock screen clock, fixing some memory bloat around GNOME's background handling, and other optimizations.
Hopefully all of these improvements will make it into the forthcoming GNOME 3.36 and in turn Ubuntu 20.04 LTS.