JPEG, and by extension MJPEG, is far simpler to decode than most video codecs, and also massively parallelisable. 4k at 60FPS is less than 500MP/s, which is easily achievable by a GPU.
Sorry, but I've benchmarked this recently as part of a project that I'm working on, and JPEG decoding with libjpeg-turbo consumed far more CPU than software H.264 decoding.
The encoding side is very expensive with H.264, but as I understand it a lot of work goes into building the right reference frames so that higher compression and faster decode can be achieved.
