Is normal that performance of my SSD drop when the other SATA port device is being used hard? Is that because the SATA controller is more "stressed"?
Yes, I believe that is the performance limitation of the AMD SB750 chipset.
You should be able to increase your SSD performance by increasing the voltage to the South Bridge controller, but I don't know much about overclocking so you'll have to do some research on how to do it.
Also, updating your AMD South Bridge & AHCI drivers to the latest version may improve performance.
Here's the link: http://support.amd.com/us/gpudownload/windows/Pages/raid_windows.aspx