What Does This Mean In The Real-World?
While our synthetic tests show throughput performance scaling, that's not very applicable to real-world use. If you exclude thumb drives, most enthusiasts only use one external storage solution at a time. And those who have more than one probably don't max them out when they're attached to the same controller, at the same time.
In that context, you're most likely going to hammer performance when you issue multiple commands to that one attached drive, such as if you write two files at the same time. We don't get NCQ support, so you're always dealing with a queue depth of one. At the driver level however, the operating system interweaves commands using a memory buffer. This is what allows you to read and write to a USB device at the same time.
Practically, you see the speed of a write operation slow down for every additional write that you pile on top. However, the sum of file transfer speeds should still equal what you get from a single transfer.
Reading combined with writing is handled a little differently. Take a look at our two videos. In them, we're playing a raw Blu-ray rip (30 GiB) and writing to the same drive.
Due to the availability of separate buses for transmission and reception, reading and writing simultaneously occurs just as fast as it would if you performed each task separately. That's why we don't see a sustained drop in write speed, and video playback doesn't stutter. The only difference between USB 2.0 and 3.0 is the speed at which the file transfer occurs.