You need to enable debug logging in the server settings.
Then inspect the Plex Media Server.log to see what is being done at this moment.
There could be 2 potential reasons:
- You have video preview thumbnail creation enabled for this library, which requires that the file is being read from start to finish https://support.plex.tv/articles/202197528-video-preview-thumbnails/
- You have requested a detailed media analysis, which also requires reading the file from start to finish, because it determines the real bitrates of the various media streams in the file. That is something mediainfo cannot do at all. mediainfo only tells you the average bitrate of a file, which is quite meaningless with video streams which can have a variable bitrate. A bit more info is in here: Bandwidth calculation error
