Array to array copies with Unraid were never fast, mostly because of parity being a dedicated drive, with earlier releases it was a little faster with Turbo Write, still because one disk has to read and write at the same time it was never that fast, but you could get 70/80MB/s, now, and since the write mode auto changes to r/m/w it can never go above 50/60MB/s, so what you are experiencing is normal, and speed will be the same with or without turbo write, turbo write is much faster for any transfers originating outside the array, like your desktop or from cache or any UD device.