Use Plex for Windows.
If you want a big screen TV layout (like TV Mode in Plex Media Player), try Plex HTPC, the new client for home theater PCs.
Your system is transcoding the video.
Transcoding video can use hardware acceleration via the RTX 2080. However, HDR tonemapping on Windows does not support Nvidia GPUs and will occur on the CPU. HDR tonemapping is also a preview feature, so even with a powerful processor such as a Ryzen 5600 it may not work well.
Plex Web cannot direct play HEVC video due to web browser limitations.
Plex for Windows (PfW) is a more capable client than Plex Web. PfW uses mpv.io as the underlying video player. It can scale 4K video to lower resolutions and tonemap HDR to SDR without involving Plex Media Server. As a result, using PfW, you can direct play 4K HDR media when using a 1080p SDR display.
If you do try Plex for Windows, check Settings → Plex for Windows → Player (Show Advanced). In the video section you can adjust Video Playback Quality and also enable Hardware Decoding. Higher quality settings use more system resources (CPU, RAM, etc). Hardware Decoding lets Plex for Windows decode video using your Nvidia GPU instead of the CPU.
