Hello,
I have an Intel 520 80Gb SSD drive.
If I look at advanced info in the Intel Rapid Storage Technology program, it says my transfer rate is 3 Gb/s, and I confirmed this using ATTO disk benchmark, as I'm getting read rates capped at 250Mb/s.
I'm running Windows 7 64-bit on an Asus P8Z68-VPro motherboard, and Intel i5 2500k.
I have installed all chipset and sata drivers, as well as the latest BIOS and the latest firmware to the SSD (using Intel SSD Toolbox).
I'm also 100% sure that the drive is connected to the correct SATA-port, it is listed as connected to the first SATA3-port in the bios.
AHCI mode is turned on in BIOS.
I can't find anything that causes the slower rate, and have come here looking for help 
Thank you