If the card is really slow, and it was cheap, it could be a fake, but don't jump to conclusions quite yet. I once bought a 16gb Class 10 SDHC card, and when I couldn't achieve the 30mb/s read (or anywhere near it) I contacted Sandisk.
I then spent the next week or so sending them pictures, serial numbers and other details such as where I bought it from and it took them a surprising amount of time to confirm whether it was genuine.
Once confirmed it was genuine, although I had a new sandisk multi-card reader, I had to get a specific sandisk sdhc only reader to achieve the fabled 30mb/s. Sandisk allowed me to send my current one to them and they sent me out one that would work to it's maximum capacity.
All this, just to say you can't trust the speed tests you get from windows or osx, as even when you have genuine cards and genuine readers, sometimes there's more to it than that. When I contacted Sandisk I mentioned that they should make it more clear, as nowhere do they say that you need specialist readers to get the best speeds!