The size of the file does not matter.
What matters is the bandwidth required to play the movie.
You can see the AVERAGE bitrate in the Plex web interface by a) sorting by bitrate; b) looking at the Get Info for the movie; or c) analyzing the file using a tool such as MediaInfo. FYI, A & B are also available in Plex for Win/Mac and Plex Media Player. Other clients may also have a sort by bitrate capability.
There are two issues:
First, the network connection for the TV.
Your TV most likely has a 100 Mbps Ethernet port (I’m not aware of any with a 1 Gbps port). High bit-rate videos definitely burst over 100 Mbps. When this happens, your TV cannot receive data fast enough, so playback pauses while the buffers refill. Once enough data has been buffered, playback resumes.
The 5 GHz WiFi in your TV most likely supports connections faster than 100 Mbps via 802.11ac. If you’ve a compatible WiFi router/AP and a strong signal, there is a good chance high bit-rate videos will play without buffering.
As a reference point, I’ve a LG B7 OLED. High bit rate videos buffer when it is connected via Ethernet cable at 100 Mbps. The same videos do not buffer when it is connected via 5 GHz WiFi.
Second, regarding subtitles, audio formats, quality, and transcoding (they’re interconnected):
Part 1:
When using the Plex LG app:
- Enabling image subtitles, PGS & VOBSUB, results in Plex transcoding the video.
- SRT (text) subtitles, direct play.
- However, if the audio is transcoding and SRT subtitles are enabled, the video also transcodes.
Part 2:
You don’t mention your TV model, but recent LG TVs support Dolby Digital, Dolby Digital Plus, dts, and dts-HD audio. Audio tracks in these formats should direct play.
TrueHD is not supported. If you play a TrueHD audio track, it will be transcoded by Plex Media Server to a supported format (reference bullet point above regarding transcoded audio and subtitles).
Part 2:
When transcoding video, Plex Media Server transcodes to H.264, 8-bit SDR video. Furthermore, Plex Media Server does not tone map HDR to SDR. As a result, transcoded HDR video generally looks quite poor, with washed out colors.
Part 3:
What it boils down to is, when playing HDR video:
- Avoid PGS & VOBSUB subtitles, as they force a video transcode.
- When using SRT subtitles, avoid TrueHD audio, as the combination results in a video transcode.
- You’ll know when you’re transcoding HDR video, as it will most likely look quite poor.
Transcoded SDR video should be OK. With your CPU and hardware acceleration it should have good quality and minimal buffering.
Conclusion:
High bit rate video can buffer even when direct playing. If possible, try using 5 GHz 802.11ac WiFi to obtain a faster connection than 100 Mbps wired Ethernet.
Keep in mind the effect that subtitles and transcoded audio can have when using the SmartTV app. When playing HDR movies, avoid image based subtitles and TrueHD audio. Note that Blu-ray movies with TrueHD audio also include a Dolby Digital track. Choose it if possible to avoid audio transcoding.
Here’s a thread with additional information on 4K, transcoding, etc.
