Extremely slow card reader (write speed: 20 mb/s)

Hey Folks,

I got a MacBook Pro with Retina Display and an internal card reader. I bought some SD cards with speeds of 45 mb/s (write) and 90 mb/s (read). I tested them on a external USB 3.0 card reader and so I got nearly this speeds, but the internal card reader of my mac reads with 25-40 mb/s (it's ok), but writes only with 15 up to 20 mb/s.

I already checked the link speed in the system information: 2.5 GT/s, so it has to be connected with PCIe and not USB, but I can't figure out where the problem could be.

Any suggestions?