I’ve noticed that frequently the Plex transcoder is running at 300% CPU on media that is newly added to the server. I do not have optimized versions enabled, so there’s no reason the transcoder should be processing anything on these files other than detecting the media type.
The transcoder will run if you have Chapter Index images turned on at the library level.
It will scan each individual file and extract one frame, writing the BIF file, as it processes.
To disable, Edit each Library section - Advanced Tab. You may wish to delete the Thumbnails already created while there