How can I optimize duplicate photo detection on my Mac?

zman of Illinois wrote:

Obviously I have little recourse but to wait it out, else spend the money for another app. Thanks for your help.

Now that you've mentioned it, you want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the these two apps with the following results and found them to be safe to use:

PowerPhotos - $30  - Free version available with limited features.

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $15 - Demo version available.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

PowerPhotos is the most powerful and useful utility for a Photos Library. It does a lot more than just find duplicates. It can merge libraries, view libraries in different views that are not available in Photos. There's a free, demo version that does all by find duplicates and merge libraries. You can try it out to see if it might be something you'd like to have.

You're not any more impatient than the rest of us, but comparing 20000 pictures, each to all the others, takes time. Keep the computer plugged in, on, and with Photos in the background all night for another week and see. You may need to tell it not to sleep, though there are differing opinions about how important that is. Can't hurt.

PowerPhotos and Photos are looking for different kinds of duplicates.

PowerPhotos is finding duplicates based on the file attributes, filename, size, etc. That can be done quickly. But Photos is also looking for similar photos. For that it needs to compare what a photo is showing and use AI tools to recognize the objects in the photos and the scene. That will need a lot processing power for each photo and video. And Photos will only begin with the search for duplicates once all items have been analyzed.

zman of Illinois wrote: "scanning will continue when you're not using the app." I interpreted this to mean that Photos was not to be active. Your comment suggests the app should be active.

There seem to be differences of opinions about whether Photos should be closed or not. My guess is that it doesn't matter. The "not using" is different -- you should not be using the app, and if it's open, it should be in the background.

There's quite a bit of voodoo associated stuff like this, not unlike baseball. People who see that it syncs better on a day when they're wearing a a particular pair of socks will wear those lucky socks every time they try.

Plug it in; turn it on; leave it alone.