I've read the forums, and some people claim that MicroSDs are one idea slower than the SD (SDHC but anyway).
I guess that the Micro versions are slower due they have an extra layer of conductors. I mean - in SD cards the memory chip is connected straight to the pins.
But in Micro SD cards the memory chip is connected first to the pins of the small "case", then you put the small card in the adapter and you add a second layer of conductors/pins (they have their own resistance and when you double it - signals flow slowly).
Maybe I'm wrong but that's how I explain it to myself.