In any case I have tested the SD Slot on many laptop and even Efika MX Smartbook and the speed is the same, I have tested cheap Class 10 SDHD and MicroSD and none mine arrive over 20MB/s .
In any case is more fast put them on USB slot (with usb key the have SD slot) this happen exatly on every Laptop that I have tested.
of course the actual speed will be limited by the card you plug into it. I wish most sd cards were actually fast enough to reach the max limits you just posted.
But my question was the other way around. There are already sd cards out with claimed read/write speeds beyond 50MByte/s and it gets quickly very expensive, so I don't want to waste my money on a sd card which is too fast. The sd card I use at the moment reaches 15MByte/s read speed.
Edit: removed dumb claims