Like @trurlmentioned this is the result of having shares set to "most free" allocation method, parity writes will overlap making the performance considerably slower, note also that since v6.8.x "turbo write" will be disable once multiple array disk activity is detected making performance even worse, you'll see the same behavior with v6.8, I never recommend using most free as an allocation method precisely because of this performance issue, it can be a little better if used with some split level that avoids the constant disk switching for any new file.