I'm a bit uncertain by just how much faster USB 3.0 is over USB 2.0. I've read a few articles where it was stated that on paper 3.0 is 10x faster than 2.0, but would never be achieved in the the real world. The "real world" improvement would be more like 3x.
Now, I'm sure this is making several assumptions, such as native USB 3.0 and no hard drive involved in data transfer.
If you are doing a data transfer test with USB 2.0 and USB 3.0, they're both dealing with the exact same environment. If you've got a SATA II (3gb) 5400rpm drive, both speeds will be hampered if the hard drive is involved in a data transfer. Also, USB 3.0 is enabled on my laptop via a USB 3.0 port plugged into an Express Card slot. The device is rated for USB 3.0, 5Gbps, so I don't think it's a bottleneck.
I conducted a test with both and found only a 42% improvement. While still significantly better, it isn't quite as stellar as I'd have expected. In the test I copied the same files from my hard drive to a USB 3.0 enabled external hard drive (also supports USB 2.0).
What I'm trying to understand is if there's another bottleneck somewhere. Does the very fact that I have a SATA II 5400rpm hard drive end up capping USB 3.0 but not USB 2.0, so that the difference is less? Is there another kind of test I can do to bypass any data touching the hard drive? I wonder if I can download a file from the Internet, specifying the external USB 3.0 drive, and that there wouldn't be any caching on my slower internal hard drive to interfere.