I seem to have been having the same issue for a couple of years now and can't seem to find a fix for the main catalogue, which has meant me not doing slideshows for wedding clients during this time, which is a shame.
LR Classic 7.1
Mac OS 10.13.2
Come on Adobe. My loyalty is waining. Maybe I should just moved to Capture One as friend have recommended?
I just bought an iMac Pro and, aside the disappointment of the general speed and the headaches to activate/deactivate my Lightroom Classic CC v.8, I also incurred in the famous (or infamous) slideshow export.
I have tried several scenarios (only landscapes, adding portraits, removing stars, adding and removing "zoom and pan"...) and I ended up finding that if the transition time was 2 seconds, the system was hanging, if I modified this (i.e. 1.9 sec or 2.1 sec) it worked flawlessly.
But I must add that on my NoteBook Pro (same OSX, same Lightroom Classic CC v.8, same catalog etc.) the export worked perfectly EVEN with the 2 sec transition time.
Now, the only REAL question is why Adobe is not fixing this bug (as Matt Boadle said)
Let's keep on searching...
The second solution is quite ridiculous: 1. Install the program to dump the image and sound from the screen in the form of a movie. 2. Start recording in the program to dump from the movie screen. 3. Turn on Play in the presentation in LR 4. After recording, set the beginning and end of the movie. The conversion of the presentation will take a dozen times less time than with normal conversion. Why is it so, I have no idea.
- use iMovie to create a slideshow (which can take longer), or
- use Mojave's built-in screen recording feature (Shift-Cmd-5) to record the slideshow preview (so I don't have to insert captions etc in iMovie), and then import it into iMovie to add an audio track (if required).
Adobe - I expect my subscription fee refund until you have fixed this.
Creating a slideshow in Lightroom never works. It hangs forever while displaying "Export Slideshow as Video" in the top left corner. Many users have reported this problem for YEARS:
Used it solidly for a day and it was progressing as expected. Then after adding some more images (HEIC and some jpegs) it started to hang. Quit and Restarted 3 times and then it finally started working again.
The status information needs to be improved so it is clear what is going on. If there is a pre-roll processing step that makes up the time, then then drilling into the status should say that. Then it should really say processing image number XX of YY. Add some more feedback and quote how much has been written to disk. Be even more helpful and start to estimate the time that this will complete. Currently it just says exporting. If there really is an issue making it to the point of writing to disk - report that there is an error.
I suspect there is some engineering opportunity to speed this process up with some parallelisation. I don't see a lot of cores lighting up when slide show is being rendered.
My only solve was getting the graphics card software update, but I wouldn’t be surprised if other issues might hang up the process. Love the idea of getting better status info and error messages when something goes wrong.
1) It appears a previous bug when using Portrait and Landscape images with Option> Zoom to Fill Frame checked is back (or was never fixed). Try unchecking that option and then restarting LR.
2) In the Library module select all of the image files to be used in the Slideshow. Go to menu Library> Previews> and select 'Build 1:1 Previews.' This fixed the issue with landscape and portrait images using 'Zoom to Fill Frame' checked so it may also fix your issue.
3) Update your graphics driver by going to the manufacturer's site and downloading the latest drivers for your GPU.
BTW I'm having no Slideshow issues using LR 9.2 on my Windows 10 version 1903 system. That includes mixed landscape and portrait images, 'Zoom to Fill Frame' checked and 1080p video output.