I have four Samsung 850 EVO 1TB SSDs in a RAID 10 array on an ASUS Prime Z370-P motherboard using IRST 126.96.36.1993. It has run great for two years, then array access slowed. In the IRST app one of the SSDs shows a SATA transfer rate of 3 Gb/s (it originally showed 6 Gb/s; the others still show 6 Gb/s). Restarting the computer makes no difference.
Any idea how to fix this?
Well, first thing I would be doing is displaying the drive's information via a different application (just in case it is a bug in the application). You could try AIDA64; it lets you try without purchasing.
If you think it is a physical issue, first thing I would try is a replacement SATA cable. Second thing is moving the drive to a different SATA port. If this doesn't help, I would be thinking about replacing that drive.
Remember that, with this particular system powered down, you can temporarily remove this drive from the system and connect it to a system without RAID enabled and check its (perceived) specs and performance.
Hope this helps,