SATA3 recognized as SATA2 on Dell XPS 9530

Hi,
I bought one year ago a Dell XPS 15 9530 with 1 TB HDD + 32 GB SSD mSATA. Recently I bought a 500 GB 850 EVO SSD (SATA3) and replaced the HDD. Then I installed the original windows version using the recovery USB key. Now the problem is that when I open Samsung Magician it tells me that the SSD is not connected to a SATA3 port, but to a SATA2.

Why? I checked the specs and used some other program to check and all the ports are SATA3.
Also the benchmark gives as a result a speed that is half of the one expected (with a SATA3).

I unistalled the Intel Rapid Storage Technology software and drivers and used the Windows ones. By doing so Magician recognized the port as SATA3, but the computer boot was 5 times much slower and the benchmark of Magician didn't work that well.
I tried SSD-z to benchmark the SSD and with IRST it had a sequential read speed of 230 mb/s while without it is 460 mb/s. Also the read access time is much shorter.

Can someone explain to me the reason of these odd behavior and also how to fix both the problems:
-boot slow without IRST
-half speed of the SSD with IRST

Thanks

 

I found out checking the Event Viewer that I had an ID Event 153 (disk) every few seconds and occasionally a 129 (storahci.sys). The event 129 exactly when the computer freezes during the boot and while working. These events don't show up while using the Intel Rapid Storage drivers, but these the SSD runs much slower (230 mb/s instead of the 500 mb/s).

Can anybody help?