The Scalar 218 comes with a choice of 1 or, in your case, 2 of DLT2000XT,
DLT4000 or DLT7000 drives. The DLT2000XT has a 75MB/min native transfer
rate (300MB/min 2:1 compression) which translates to about 4 gig/hour. If
you're using this one, you might want to upgrade to the  DLT7000 model
which is 4 times faster.

About the tape copy problem...the reason may be that tape-tape copies
could
take twice as long as transferring data to tape because the data has to go
through the server bus twice. Data is read to server memory then written
back to tape.
Some systems allow direct tape to tape copies without the data having to
go through any host CPU but are quite expensive.

Finally, one other reason why tape backups may be slow with ADSM is
because
ADSM does not support saving multiple client data streams to tape
(although
it does support multiple streams to disk). So your tape drive may not
always be driven at its maximum speed during backup.