btrfs locking at 74% on 6.1.6

I have been noticing similiar behavious myself where my load will spike up to 30 on 6.1.6, but since I never did a factory reset yet I have just begun the process of debugging what is wrong although I think my problem is related to installed apps though. I am impressed that my NAS is not crashing though.

Have you tried turning off all addons while copying?

One thing I noticed in regards to your top imagaes you provided is that Plex Media Server is running when you have these problems. I have also noticed my Plex Media Server seems to be high in the top list when I get these spikes. When I turn off addons like plexmediaserver and nzbget my load immediately goes down.

I switched to nzbget since I thought sabnzb was taking too much processing time when idle but it may also be causing problems. I also switched from using deluged to utorrent and utorrent is way better on my system load. I also created custom init.d scripts for alot of my apps that set ionice, nice and io scheduler class. For me I have narrowed down my misbehaving apps to either plexmediaserver, nzbget or bitcasa.