I solve problem recreating slideshow.
I have had the same problem with LR 6.6. I have let it run for several hours and the progress bar hasn't registered any progress.
I'm running on Mac OS X 10.11.16. The CPU on my quad core had never gotten beyond 6% for Lightroom. It should be much higher.
I don't want to recreate the slide show if I can help it.
I also progress bar is frozen for hours. only 72 photos and 2 mp3? help