http://jell.yfish.us/ using these sample files the largest 400Mbps HVEC (h265), runs fine on movies & tv (w10 app) and Kodi from 7200rpm HDD, SSD, NVME raid, HDD raid, and ram disk.
VLC, mplayer and windows mediaplayer are crap for playing HVEC and not so sure if they are that good for 4k h264 either (most 4k are HVEC encoded in the first place). Threadripper does not have integrated GPU, so for hardware support you need player that support using your graphics card as the hw decoder or CPU with integrated GPU. Software encoder aren't that great for large datarate files. It has nothing to do with disk speed, caching should take care of that for playback.
...and for second time NVME is supposed to run at x4, there is no faster NVME in any MB and NVME modules don't support fasten than x4 PCIe lanes. You are confusing NVME to the buggy PCIe x8/x4 setting in BIOS.