[6.7.x] Very slow array concurrent performance

Since I can remember Unraid has never been great at simultaneous array disk performance, but it was pretty acceptable, since v6.7 there have been various users complaining for example of very poor performance when running the mover and trying to stream a movie.

I noticed this myself yesterday when I couldn't even start watching an SD video using Kodi just because there were writes going on to a different array disk, and this server doesn't even have a parity drive, so did a quick test on my test server and the problem is easily reproducible and started with the first v6.7 release candidate, rc1.

How to reproduce:

-Server just needs 2 assigned array data devices (no parity needed, but same happens with parity) and one cache device, no encryption, all devices are btrfs formatted

-Used cp to copy a few video files from cache to disk2

-While cp is going on tried to stream a movie from disk1, took a long time to start and would keep stalling/buffering

Tried to copy one file from disk1 (still while cp is going one on disk2), with V6.6.7:

with v6.7rc1:

A few times transfer will go higher for a couple of seconds but most times it's at a few KB/s or completely stalled.

Also tried with all unencrypted xfs formatted devices and it was the same:

Server where problem was detected and test server have no hardware in common, one is based on X11 Supermicro board, test server is X9 series, server using HDDs, test server using SSDs so very unlikely to be hardware related.