No two tapes are always going to be the same. Tapes will exhibit soft errors - blocks of data that are not written successfully and automatically rewritten. This is one of the reasons why we don't simply write the same stream to both drives, but rather create to self-contained streams.
As to the speed, while large numbers of soft errors will contribute slightly to performance issues, the source drive's speed can be a real issue when 2 processes are reading at the same time. Is your media drive a fast SAS or F-C array or a USB, Firewire, or internal drive?