On Sep 14, 2007, at 10:44 AM, Strand, Neil B. wrote:

I have observed almost no performance degradition using encryption.

The TS1120 redbook agrees:
"Enabling encryption incurs a less than 1% performance impact."
where the added time is to get the keys at load time.

But I think that's in ongoing operation.  From a reseller
presentation, their knowledgeable technician cited a 25 second extra
time when the tape is re-used as scratch (written from load point),
in that it has to write encryption information at the front AND
position about 1/3 of the way into the tape to additionally write the
same information there, as insurance in case the leading portion of
the tape is mangled in one of the drive problems we know so well.

   Richard Sims    http://people.bu.edu/rbs/ADSM.QuickFacts