The size of your project will have a direct impact on time necessary to back up and shut down Scrivener, so whatever is making your project larger will also make your backups take longer. See this post that explains what Scrivener does during the backup process.
Even 600k+ of words would likely not not cause very long backup times, so the cause is likely all your images.
As an alternative to storing your images directly in the Scrivener project, you could keep them in a folder external to your project and point to them from within Scrivener using Insert > Image Linked to File.
The advantage of this approach is that your project will be smaller in size, your backups shorter in duration, and your zipped backup files themselves will also be smaller in size.
The disadvantage is that you will need a separate process to backup your images. (You should already have a separate process to back up your entire PC, so in that case that would also backup your images.)
Best,
Jim