A couple of years ago I was doing a mass copy of files from one SSD to another. It was a few hundred GB, not terribly big on modern machines but it did have a large number of tiny files. Windows was doing the copy but it was estimating that the whole thing would need 8 hours to complete, and the estimate was pretty solid after 20 minutes. I cancelled the copy to investigate and tried turning off Windows Defender (but only temporarily as you said) and restarted the copy. It finished in 35 minutes. Probably would have been even faster if I didn't have one of the drives hooked to an old USB->SATA adapter.

This is also why your browser will stall out when it finishes downloading a large file. Windows Defender kicks in an does a full scan before returning from the close call.