Maybe the chunk size you are writing to tapes.   Keep that to 2GB on the 
storage unit.  Otherwise I guess you can have very long sections written 
by one of the multiplexing clients.   Is the delay (a) before any data 
comes off the tape (b) after you reckon it is all off, keeps looking down 
the tape for more or (c) just low data rate off the tape?

Maybe a bad tape, with lots of soft errors.
Maybe a DLT tape that has lost the directory at the BOT, so cannot fast 
position.  Can't recall just how you fix it, but the symptom is a tape 
that takes forever to position to the start of the data, i.e. may take 40 
minutes to  get the first block off the tape, instead of the usual 2-5 
minutes or less.

You did not say if this was a particular restore of one tape or if all 
your restores just went over a performance cliff.

William D L Brown
Ext Tel: +44 1438 762681 Fax +44 1438 762336 Mobile: +44 7717 800241