In my experience this new feature was enabled by default (!) and the Transcoder that’s doing the job is not honoring the Transcoder temporary directory setting. So, since it’s now using the OS default temp directory instead of the one I’ve specified to PMS, my OS drive got hammered slowing my system down to a crawl.
Not a very nice way to find out and can cause a bunch of issues as we’re now seeing in various threads. Sloppy roll-out by the Plex Team, but they seem to be on the ball to fix the problem.