In k-lite codec pack, there is a "codec tweak tool". Use this tool to make sure it's LAV Video that handles your video playback. (You will see the little LAV icon in the lower right corner, if done it correctly.)
By default DeoVR is using Microsoft's own decoder to handle HEVC, which is not efficient. Use LAV splitter instead.
For LAV video decoder, make sure you use "Nvidia CUVID (old)" as the default decoder, DVXA2 might work fine, but I find the good old CUVID works the best.
