When Windows UpTime hits hour h, the performances of the onboard raid is just falling down.
Seen it with different mobos, all based on Intel Chipset ICH9-ICH10,...and with different Intel Matrix Storage versions (all above 9. will do, I don't remember clearly with 8.8 or 8.9)
Simply run a HDD bench test at "right time", you will see
Am i missing some special setting in the software / hardware ?
is there any known workaround / fix for this issue ?
You can try RST but I don't why your performance would drop when it hits an hour.
Just install that.