In case you need some ideas for more speed tests to run, I just had a thought for you:
Currently the “share size enforcement” is disabled in Rockstor, and with it the “quota” of btrfs.
I found this short discussion about re-enabling the quotas at some point soon, due to progress in btrfs kernel:
I actually have the quota enabled partially on my system (on a SSD) for a while now and everything seems fine and without a noticeable speed penalty.
In case you have some time to spare and would like to do this, feel free to make an explicit test with quotas disabled & enabled and check whether there is a measurable difference.
Cheers
Simon
PS and thank you for the link regarding BTRFS Raid 5/6 @Tex1954