Yes, I recently experienced the same when I wanted to nuke 40 000 images from iCloud.
I first tried to delete them via Sequoia Photos.app but "Syncing..." remained on the Photos.app screen for hours even after an initial 100 image delete test.
Granted, just cancelling iCloud Photos might have also removed them after 30 days especially when I reduced iCloud from 2 TB to 200 GB for just iCloud Drive which works OK).
Anyway, I then decided to test if deleting from the upstream iCloud source is faster and went to the iCloud web site and deleted Photos there.
But it is possible to delete only in 1000 image batches and after each delete the user has to wait about 30-60 seconds for a "save". And the same happens also when completely deleting the images from "Recently Deleted" where also only 1000 batch is allowed and it also lasts about 30-60 seconds for each batch.
Maybe this is a safeguard preventing people doing a stupid thing to delete all images en masse which they will regret the next second...