Hey Lmankoff,
Thanks for the update. This article has more information on exporting slideshows on the Mac.
"You can export slideshows to share with others. When you export a slideshow, Photos creates a movie file with the video format that you select.
- In the Photos app on your Mac, click a slideshow under Projects in the sidebar.
- Click Export in the toolbar, or choose File > Export > Export Slideshow.
- Enter a filename for your slideshow movie in the Save As field.
- Click the Format pop-up menu and choose a file type for the exported movie.
- Standard definition (480p) is good for standard-definition TV. If the slideshow will play on a high-definition TV or display, choose one of the high-definition formats: 720p or 1080p.
- Choose where to save the exported slideshow movie, then click Save."
Export photos, videos, slideshows, and memories in Photos on Mac - Apple Support
Let's get a little more information to help narrow this down.
Is the export freezing, or is the slideshow freezing?
If you choose a different file type or definition, will it still freeze?
Do you have other options of the location to export to?
Keep us posted.