You are suffering from what could be called I/O amplification at the RAID level. For every block you write to the array, you will generate a total of 1 read from another drive and then 2 writes to 2 independent drives (data and parity). So for that single block 3 drives must be active.
BTRFS then must do the same thing for all its processes (copy on write, inode tree etc).
All of the above could be made much worse if you have SMR based drives.
As for a remedy, apart from rebuilding your array to use a simpler RAID structure and/or file system, there is not much you can do. The best option would be (if you can), purchase a good USB drive and attach it to your DS920, using EXT4. You then use this for Storj until the drive fails.