XPS 15 9520 RAID 1

I have an XPS 15 9520 that had a single drive when received from Dell.  The settings in BIOS were with the RAID enabled, everything worked fine.

The the hard drive died, as a precaution the single drive has been replaced by a pair of Crucial P310 drives which I have configured as RAID 1 (mirror).  I initially did this through the BIOS - or at least the tool you access from the F12/BIOS area.  When complete I noticed the performance was pretty poor.  I check an identical 9520 (we have 8) that had the single original drive (configured again as RAID from factory - all 8 machine are), and performance was less than half.

I then undid the RAID set up using the Intel tool in Windows (no BIOS changes) and tested and a single Crucial drive marginally out performs the original drive in the other machine.  Confused, I recreated the RAID 1 using the Intel tool in Windows, rebooted and tested again, and once more the performance is less than half of a single drive.

All updates BIOS and OS (Windows 11) are applied, so what's the story?  How are you meant to do this, because a RAID setup that slashes performance by that much isn't worth having!

Tom