Crucial p5 2tb Problem

I got Crucial p5 2tb nvme ssd on asus crosshair iv formula via m.2 to pcie adapter using as game instalation ssd.
Problem is when i put ssd to pcie x16_2 is detected as x1 in x16_3 as x4 dont uderstend why?? x16_2 is x8.
i got realy low perforformance with around 1.6gb/s read and write. i know is pcie 2.0 not 3.0
but been expecting something closser to 2gb/s. maybe should i try different nvme drivers?