Based on your Plex Web post, it seems your system is having trouble transcoding VC1 video.
VC1 video direct plays on a Nvidia Shield (Android TV).
VC1 video transcodes on my Pixel 3a XL (Android mobile).
Does H.264 video at approximately the same bit rate play smoothly on your phone? You can sort by bitrate in the app to find a video with a similar bitrate.
Check Settings -> Server -> Transcoder. Make sure both Use hardware acceleration when available and Use hardware-accelerated video encoding are selected, and Disable video stream transcoding is not selected.
Play a VC1 video without subtitles on your phone and monitor playback in Plex Dashboard -> Now Playing via Plex Web.
Is the video transcoding? If so, do you see a (hw) for both the decode and encode?
