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UHD is HEVC (High Efficiency Video Coding) aka h.265 which in essence means you double the compression of the data but produce the same video quality even though it's compressed.  Otherwise a video file on a physical disc or your HDD would be about 150GB - 200GB.  At the same time, using the same bitrates we are accustomed to, the video quality is improved significantly.  Now you need to decode it.  For that you can't use the decoder you were using previously (h.264).  You need an updated one.  HEVC aka h.265.  Furthermore, a CPU can't handle this computation because it is trying to SOFTWARE compute it.  To do it with ease, you want to HARDWARE decode it.  To hardware decode it requires HEVC ability and a sufficient amount of vRAM (not memory in your PC but the memory your GPU is accessing nor CPU strength since it is not doing any decoding) proportionate to the difficulty of the task.  Those difficulties include bit rates and refresh rates amongst other things.