SD card class 10 transfer rates slow

I have purchased a few Transcend Class 10 SDHC cards that I use for record HD video. Some of the cards are simply used for file transfers between Macs. Despite the advertised minimum transfer rate of 10MB/s, I am only achieving 2-3MB/s for copying to or from the SDHC card. I've tried formatting the card both on the Macbook Pro using disk utility, and also on a PC card reader to see if there would be a difference in performance. The PC formatting to FAT32 seemed to help the transfer rate (now 3-4 MB/s) but I am still not within the region of the advertised rates. How can I speed up the transfer rate, or is it the SD card reader itself (revision 1.0) built into the laptop that is the bottleneck?