It's shocking how much performance you gain by temporarily turning off Windows Defender. I had a local disk to disk copy that the Windows File Transfer dialog estimated was going to finish in 10 hours after it settled down. It wasn't even that much data, just a few hundred GB, but that consisted of a great number of small files. On a hunch I tried disabling Windows Defender and the estimate dropped to about 30 minutes, and in the end it was spot on.
Anti-virus kills small file IO. I work with a windows product that can deal with huge amounts of code files. With large enterprise that demands AV is on in most places the performance loss is pretty staggering, were you can easily lose 50% of your servers capacity per measurement interval.