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.