The file size will be humongous, but the movie will look awesome. Choose this option for a stunning 1920 x 1080 pixel movie, which is what you want for viewing on the latest iOS devices, Apple TVs, and TV screens. It’s perfect for newer but not the newest iOS gadgets such as a 4th generation iPod touch, iPhone 4, iPad, 2nd generation Apple TVs, for posting onto Facebook or YouTube, or for sharing via file-transfer service like iCloud Drive or Dropbox (there’s more on those services in a bit).

This option produces a 1280 x 720 pixel movie, which makes for a larger file size but it looks fantastic onscreen. It produces the smallest file size, so it’s a good choice if you plan to share the slideshow with someone who doesn’t have high-speed Internet access (gasp!). This option produces a 640 x 480 pixel movie, which is great for transferring onto an older iOS devices (iPod touch 3rd generation, iPhone 3GS, iPad 1, and 1st generation Apple TVs). After opening it, you'd click on Memory, then organize it by what's using the most.Standard Definition (480p).

You can open Activity Monitor by clicking on the magnifying glass logo at the top right of the screen, then type in 'Activity Monitor'. You can check within Activity Monitor to see what's using the most memory.Another good idea would be to try exporting your slideshow to another place other than the one you're currently trying.

Have you already tried a restart since this began to occur? Check out the section 'Restart' here: Log out, sleep, wake, restart, or shut down your Mac.

Let’s make sure all of your apps are up to date: Update apps or use automatic downloads.

Here's how to first Back up your Mac with Time Machine, and afterwards, here's How to update the software on your Mac. You can verify which version number is installed by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’.

You mentioned having macOS Big Sur installed, but the latest version is macOS Big Sur 11.1.

We'd like to help get your slideshows exported without any issue, so let's start with confirming some basics:

Mac slideshow app