In order to remove the failing disk, you are going to have to stop the copy and shutdown. You could then restart the copy with the missing disk emulated by the rest of the array. Then you could set a New Config without the missing disk and rebuild parity.
Is this what you had in mind?
If you are reasonably confident in the rest of the disks, and you have good backups of any irreplaceable files, then this might be OK. But you will be working all of the disks to emulate the missing disk, you will be operating without any protection, and it will still be slow since all disks must be read, then a data disk and parity disk must be written.
It shouldn't be as slow as what you are doing now, and it might even be more reliable. I think there is a good chance unRAID will disable the failing disk before it ever completes the copy anyway.
I often recommend not writing to any disk in the array when it is compromised, and instead copy data to another system or an unassigned disk. That way parity isn't written and there is less disk activity.