SDHC Class 10 slow transfer speeds?

[Ike]

Bought a class 10 SDHC card from

play.com, however the highest write speed I have got is around 3MB/s when it should be more around 10MB/s. Using it in my 3DS also seemed to slow down the system boot as well.

 

Is this right or is there some sort of problem with my SD/PC?