I switched one USB policy setting in Device Manager and my file transfer speeds doubled

I had noticed a strange occurrence when transferring large files on an external drive. The copy speeds start quite high, sometimes above 300MB/s, but end up collapsing to a crawl. I was seeing really chaotic-looking progress graphs, and transfers that should have taken minutes ended up taking longer. I wasted time changing the enclosure, cables, and even blamed thermals, but the real cause turned out to be a USB policy that had been the default since 2018.