The Finder on MacOS breaks when it has to deal with more than about 10,000 of anything. The MacOS Finder is chock full of O[n] and perhaps even O[n^2] algorithms that have been there, unfixed, since System 7. Back in 1996, a user with more than 100 files in a folder was unusual, so those algorithms worked fine. Nowadays not so much.
Proof: Copy 10k files from one hard drive to another with the Finder (not the same drive; you have to prevent COW). Now do the same thing with e.g. cp or rsync on the command line.
Proof: Copy 10k files from one hard drive to another with the Finder (not the same drive; you have to prevent COW). Now do the same thing with e.g. cp or rsync on the command line.