I try to do a quick slide show by selecting bunch of slides and it takes forever (minutes per slide) to go through "Preparing Preview for Slide # N". LRCC2015.5
First, update the Lightroom to CC 2015.5.1 version and try it.
If it doesn't work then try turn off the Graphics Processor from the Lightroom preferences.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor
Let us know if that helps.
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