It’s important to update the quality settings in your Plex app(s) to make sure you get the best streaming experience.
MaximumMaximumOnOnOffOriginalOnOnThese may have different names depending on the platform — anywhere it asks for quality levels, set them to Original if available or Maximum otherwise.
The settings Plex starts with are not great and will likely result in a low-resolution stream that buffers frequently.
Plex apps default to a relatively low 720p resolution setting, which means that any higher-resolution files have to be converted down to that lower quality before you can watch.
While my Plex server is working on doing that, your stream is just sitting there waiting to play.
Updating to the “Original” or “Maximum” quality setting means Plex can just send you the original high-quality stream directly without having to convert it.