I have the same problem with UNRAID v6.8 rc5 and before. I don't use plex, but all the shares become completely unresponsive when the mover is running.
I have two Samsung 860 EVO 1TB in RAID 1 (btrfs) assigned to cache.
Changing the priority in CA Mover Tuning to Low didn't change a thing.
Iowait is really high. Up to 84 sometimes.
Turbo write is disabled, but didn't make a difference when enabled.