The current version of Lightroom depends on the GPU to make the slideshow function. A lot of users, including myself, cannot use the slideshow option because we don't have compatible GPU. I have verified this by going back to Lightroom 4.4. The slideshow still functions as it should in that version on my computer. I'm wondering about the possibility of changing the dependence on the GPU. I don't know, maybe it's too difficult to go back.