The occasional micro-stutter isn't a major issue of course, but it's surprising that your GPU is hitting 67% with downscaling. For me it's around 30% with the same GPU. I use mpv player.
Haven't actually checked the 4K downscale performance since I don't have a 4K HDR TV and I only get a 4K movie when I can't find the 1080p version. 67% is for native 1080p. I don't have HAGS enabled btw.
Maybe it's a stupid question, but in the nvidia control panel you have the vertical sync option enabled, if not, do it.
I will try 4.17, thanks for the suggestion. Yes, vsync is always on.
L.E.: Tried 4.17, I can still see microstuttering. Will try V2 as well.