With iRST there is a constant verification of any RAID volume, this is part of the monitoring feature that cannot be disabled. However, it will not decrease disk or CPU performance.
Have you tested using different versions? possibly an earlier version provided by the motherboard maker.
I have a 2 year old HP, the driver that came with the PC was prior to 9.6 and does not support the 4K sector drive that I am using as disk 2 in the RAID, so per instructions I downloaded the latest version. I did not try any other versions.
For several weeks I did not have any issue. But last week with no config changes made, suddenly I started having this issue, when especially in Windows Live Mail, and also IE when trying to watch an embedded video, the drive light was on constantly and any action I would take (e.g. moving from one email to another, trying to navigate in IE or launch a new IE instance, would literally take 2-3 minutes.
I did a verify and no errors. I have since disabled write back caching as an experiment. It seems to be better for now, but not sure if that was the issue or something else.