Multiple Issues: Buffering and Subtitled Files not auto-playing

[gnattu]

Either your transcoding cache (c drive) or your media disk (f drive) is super slow so that jellyfin cannot even copy the files to its cache fast enough. The logs indicates that the copy speed is less than half of the required speed for realtime playback. As you mentioned that you are using "on two spinning platter drives.", that could be the problem.