Hard drive manufacturers are sneaky, and when the operating system tells them to flush, they often don’t actually flush. They just say, “Yeah, I flushed the data,” and make a note to themselves, “Y’know, I probably should flush the data at some point before anybody notices that I’ve been lying to them all this time.” This can lead to strange things like a subsequent Read operation taking a very long time. It’s not that the read itself was inherently slow. It’s just that the read happened to occur while the drive was busy “paying back its flush debt”, so it got stuck behind a physical flush operation.