If you attach it to a USB 3.0 port, you will get USB 3.0 transfer speeds. If you attach it to a USB 2.0 port, you will get USB 2.0 transfer speeds.
Probably irrelevant at this point in time, since the cards that you plug into it will be slower than USB 2.0, but it leaves you headroom for the future.
Edit: After looking at the mfg site, I suspect that USB 3.0 is poorly-translated hype. I didn't see any 3.0 connectors, internal or external. But it won't matter because... see paragraph 2.