Note: Please disable verbose logging in Settings → Server_Name → General (show advanced). It adds unnecessary information to the log files. As a result, desirable information may be lost as the logs will quickly wrap.
Regarding subtitles:
From Plex Media Server.log:
Jul 10, 2022 03:38:30.882 [0x7f975afb4b38] DEBUG - [Req#b0b3/Transcode] MDE: Weathering with You (2019): selected subtitle cannot be converted to a compatible format, burning into video stream
ASS subtitles must be burned into the video stream when using Plex Web. They cannot be sent as a separate stream like the video and audio.
Subtitle burning uses the CPU, even when using hardware accelerated video transcoding. That is why the CPU utilization is higher.
Subtitle burning is also a single threaded process. So, if the video bitrate exceeds the capacity of a CPU core to burn subtitles, you may experience buffering. Example: My server has a i5-10500T CPU. It can hardware transcode multiple 4K video streams. However, it cannot burn subtitles into a single 4K transcode in real time.
Workaround: In Settings → Plex Web → Player (show advanced), change Burn Subtitles from Automatic to Only Image Formats. ASS subtitles will then direct play/direct stream, but formatting information such as position and color will be lost.
Subtitle support varies widely across the various Plex clients. ASS subtitles have poor support on most clients. Expect that enabling them will result in transcoding to burn them into the video stream.
There is work underway to add ASS support in the Android / Android TV clients, but it is only available in some preview builds (See Nvidia SHIELD/Android app SSA/ASS format anime subtitles + transcoding problems ).
