If there is only one port connecting the 5 externals to the main server, then of course it is going to be very slow because the 5 disks must share the single port. This will also affect parity checks and rebuilds since all disks are used at the same time, and if they have to share a port everything will be waiting on them.
unRAID will not rebuild parity unless you make it do so. Instead, it will insist on clearing any disks you add to the array if they aren't already clear.
Deleting files or formatting will not help in any way, since those operations will not result in a clear disk. A disk must be all zeros when it is added to the parity array so parity will remain valid. Disks that have all the files deleted or that have been formatted are not all zeros. So unRAID will clear them if you don't.
It would also be possible to set a New Config with the disks in whatever state, and let unRAID rebuild parity and format them.