Very slow SSD performance on Dell XPS 13 9350

Hi

I have a Dell XPS 13 9350, with a Samsung CM871 M.2 SSD, running the installed Win 10 + updates..

Rated SSD performance should be around 500 MB/s for sequential read/write

Running the latest CrystalDiskMark, read speed is around 500, but write speed is no higher than 60 MB/s ...

I did many searches, saw couple of similar reports, but no answer. Did see one person referring to AHCI, and when I looked in the BIOS I indeed saw that the BIOS is configured to "SATA: Raid", and not AHCI. Trying to change to AHCI resulted in windows not booting, so I switched back to RAID.

Any ideas what could be the cause of this extremely slow write performance?

The computer is up-to-date with all the latest drivers, both from windows 10, and the Dell Assist / update. 

Thanks !