it only gets full speed in a few scenarios. use "ATTO disk benchmark" to see max speed. if its still low, ive seen that new drivers help, im not sure if the vertex 3 is a line that needs a firmware update. but maybe. also trim may not be enabled on your computer.
check this out for trim, the reqirements section will tell you what to enable.http://www.tomshardware.com/reviews/ssd-trim-firmware,2800-2.html