The 4 CPU Threads are a bit busy, 85% +. Which is a bit weird because hardware encoding is turned on.
Welcome to the world of Linux, where Nvidia graphics do not support 10bit video in a VDPAU driver setup, only 8-bit. Check the video processing when pressing 'o' on your keyboard, it will probably mention it uses SW (=software) decoding, and thus letting your CPU do all the hard work.
The video card itself is fine, but not fully utilized as it could/should be. Nvidia now utilizes a new driver method NVDEC/NVENC in Linux, but so far no Kodi developer has felt the urge to dive into yet another deep hole for coming up with a solution. Nvidia keeping things closed sourced is the main problem. For Kodi usage, Intel and AMD have brighter futures in Linux, next to the usual Raspberry, Rockwell and Android boxes.
So i have two questions, how can i lower cpu usage and how can i play my 4k files smooth?
That will require a Windows operating system. Just because of all Nvidia's shenanigans in Linux mentioned above, Nvidia support is planned to be dropped by Kodi (and thus LibreELEC) after Kodi 19.