Low writing speeds with SSD

Hi, i've been reading around in the forum and haven't a problem close to mine, so here it is.

I bought a Apricorn solo x2 with a Sansung 840 PRO 128GB for my Mac Pro 3.1 and installed it on PCIe Slot 2. But Writing speed it's 80/100MB/s and Reading around 200MB/s most of the times, but once in a while goes up to 400MB/s.

I've been speaking with Apricorn assistance and they recommend to plug the SSD directly to the Sata Bay. I unplugged al my other drives and plugged the SSD to all 4 Bays, but on every of them the Writing and Reading are still the same (W100MB/s / R200MB/s).

The problem is not from Solo x2, could it be the SSD or is it from my Mac?

Thanks for the support! :)