What version of Lightroom?
What operating system?
How much RAM?
How long is too long?
How many images in the slideshow?
Is this a new problem, or is it persistent or ongoing?
There is Lr CC I have 21 pictures and there is with music and the play the preview I just can see a black screen with out any image and sound and the time when I try to export takes for ever and this never happen to my I used before Lr 5 and never have any problem but now some times doesn't open the app aren't why