Linux software RAID6: rebuild slow

I don't remember exactly the speeds I had when I migrated to 6 disk RAID 6 from 4 disk RAID 5, but they were similar (4TB usable array, 24h rebuild, so around 45MB/s).

You have to remember that even the speed_limit_min will give some priority to applications that try to use the array. As such, the mechanism used to detect activity may require a 50% load on the disks to detect it and still have the ability to serve the IO requests. Did you try unmounting the partition?

To check for bottlenecks you'll have to trace the kernel (for example, using Linux Tracing Toolkit lttng, or System Tap). It's not easy and will take lots of time so unless you have to rebuild the arrays on few computers, It's probably not worth it. As for changing it: I'm sure such patches to Linux kernel will be welcome :)