Slow iCloud photo sync on Mac Studio: How to identify problematic photos?

Mac Studio, Sequoia, photos App - is not syncing to iCloud or is slow. I stays at 120 photos to sync for 30 minutes. How do I know which photo is hanging up the sync or the problem? Photos library in iCloud is about 20,000 photos.

[Re-Titled by Moderator]

Original Title: photos sync to iCloud is slow or hanging. How do I know which photo is the problem?