BTRFS scrub speed is insanely slow.

Sorry for the delay. I did some background research. 

My drive setup was unchanged from the time I initially set up the readyNAS. Also, right from the start, I put several TB of data on to the readyNAS. It has grown slightly, but I would estimate that at the very beginning this thing had about 2TB of data on it, and now it has more like 4TB, so maybe 2x growth since inception. 

2015-07-04: Running firmware 6.2.4

Scrub completed in 24 hours (this is expected performance level, result is good...)

2015-12-20: Upgrade to 6.4.1

ReadyNAS started having kernel panics, which would result in a disk check on startup, then another kernel panic, etc...

Prior to 6.4.1, never had an issue. Opened a support case, was eventually resolved with...

2015-12-29: Upgraded to 6.4.2-T59

This was a beta release that fixed the kernel panic, I think it was actually the RC for 6.4.2 and eventually became 6.4.2

Kernel Panic problem is indeed solved. 

2016-01-04: Runnign version 6.4.2-T59

Started scrub, ran for more than 30 hours and did not complete. Eventually stopped it and rebooted the readyNAS

Here is what I wrote to the readyNAS customer service at the time...

"Today I needed to restart my readyNAS. It started scrubbing the disk (which is normal, I have scheduled it to happen every few months), and then became very slow and unresponsive. I could still ping it, and get into the admin interface, but the drive itself was effectively hung."

2016-02-07: Running Firmware 6.4.2

Started scrub, ran for more than 2 days, made minimial progress, and I stopped it. 

Here is what I wrote to tech support at the time: 

"The new firmware 6.4.2 is working much better. The only thing I would note is that the disk scrubbing now seems to be causing extremely poor performance on the unit while it is underway. I have 8 TB of space on this unit, with only 2 of them used, and the scrub finished about 3% in more than 30 hours, indicating that it would take something like a full month to do the whole thing. That doesn't seem right to me. Also, while the scrub is ongoing, the unit becomes nearly unresponsive, repeatedly disconnecting, and taking ages to even bring up the management page."

So I think it's fair to say the problem started sometime between 6.2.4 and 6.4.2. Sorry I couldn't narrow it down any further.