QNAP RAID rebuild way too slow, resolved

In an 8-drive Qnap TS 879 Pro, we had a drive fail. After replacing the drive, the rebuild started automatically. A couple hours later, the status showed only 1% rebuilt. At this rate, it will take a week to rebuild – that’s no good, what if another drive fails in the meanwhile?

Searching brought me to this page that explains how to improve this. We logged into the unit as user admin and executed the following command:
echo 40000 > /proc/sys/dev/raid/speed_limit_min

Then monitoring with cat /proc/mdstat I saw the “finish” value drop from over 8800 minutes to less than 1400. Now the rebuild should be done in one day instead of one week.