I have never had a problem with the USB key and Cubase. And I ran a really slow computer in comparison with SX 3 on it. Even these days my machine isn't that fast and I don't have any troubles.
I would be interested to get the whole story like motherboard manufacturer and USB chipset other software running, other USB devices, etc. before believing any of that. Even with the iLok I don't have any problems at all.
Now, I do know some that would run a cracked version on say their laptop or home machines so they didn't have to carry the dongle around.
I mean even if you look at USB performance, it is stupid fast, no difference than having to load a sample or page out disk.
So assuming everything else is in check. Even if there was a performance increase, it would be very slight. Oh and more ammo, the "cracked" versions run a dongle simulator. So now instead of having hardware, you have a simulator running, as if that won't hinder performance as well.