In theory USB 2.0 interface provides up to 60MB/s transfer speed. On another side SD tech is newer, so it should perform better, but originally it was oriented on portable devices such as cameras, smartphones, tablet PCs and not for desktops/laptops. Class 10 SD/MicroSD card should give you at least 10MB/s. Some vendors publish their card's speeds on their sites. Sandisk even puts such specs as UHS speed class, Speed class rating and X rating on its.
Update: Tested a few generic SDHC card (c2 and c4), the result was pretty the same. Transferring up to ~800MB will give you 25-30MB/s, after ~800MB the speed drops to 5-6MB/s. Will get Sandisk soon, with 45MB/s on it.
While USB 2.0 on paper gets up to 60 megabytes/second, in reality it never reaches that. Best I have come across was about 40 or so.
That said, As the user above says, it depends on the card you have.
Also newer USB 3.0 sticks(but watch it as many are just not that fast at all) can get 50-100 megabytes per second for writes with ease(and it will only get better as time goes on). Small files tends to slow the sticks down as well.
I have seen a cheap A-DATA USB 3.0 stick that is rated at 50 write and 100 read, it gets about 40/80 and that is not too bad at all.