I have an 830 128Gb connected to a SATA3 port, AHCI is switched on and I am running Windows 7 with a Z77 Extreme4 motherboard. I tried out a few benches and my read speads are always low, well bellow the declared ones. I installed the drivers for SATA port and Intel RST that came with my motherboard I have no idea why my SSD is acting this way.
More aboutsamsung 128gb slow read speeds
Do you connect the SSD into intel SATAIII (SATA3-0 and SATA3-1)? Don't connect to the SATA-A0 or A1. Check your MB manaul.