This has been a problem across multiple version and on both mac and windows.
When I go to preview/play/export a slideshow lightroom cc hangs after completing maybe 3%. This happens maybe 1/3 of the time, and once it does I can no longer get things to work. Past tricks were a) turning off h/w accel, or purging the video cache. Neither works.
Super tired of seeing this. Between a rock and a hard place now, as MSFT slideshow creator thingy doesn't run on the mac.