You might want to go into your BIOS and check to see how the drive(s) are being recognized. It will be under Configuration, SATA. Also check the SATA MODE and see it it's set to IDE. If it is, it's set to legacy mode and will drop the transfer rate down. Use caution--If the system (Windows) was loaded with the MODE set to IDE, Widows probably didn't load the drivers for AHCI and your system will crash if you just change it to AHCI (Needed for 6Gb/s) and reboot.
See this description on how to implement AHCI after Windows has already been installed with the BIOS SATA MODE set to IDE.
NOTE: Some of the SSD drives need the MODE set to IDE to load a firmware update.