Multiple Issues: Buffering and Subtitled Files not auto-playing

[Shinino]

These may have similar causes, and I'm honestly not sure where to begin with this.

Server is a Windows 11 PC with media on two spinning platter drives.
Client is a Sony TV using the Android TV app
Both server and client are on the same network, connected to a router via ethernet.
Issue 1: Periodically shows will buffer heavily. They won't start playing automatically, and when they do play, they stop every 5-30 seconds to presumably buffer.
Issue 2: A majority of hard-subbed video files will not play automatically. Sometimes I have to change the subtitle language (or disable the subtitle) to get it to play, and then it'll take a moment after I change the subtitling back where I want it before it will pick up again. It usually plays without any issue at that point.

Where do I need to start looking first, and which logs would be helpful here? 

Server Version: 10.10.3 (slightly behind, I can update though)
Client Version: 0.18.4
Hardware acceleration is on (Nvidia NVENC) for all file types. Enhanced encoding is on as well.
Subtitle extraction on the fly is enabled.
Log file attached.

[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.

[Shinino]

Other than 'replace the drive(s)' are there any other options?