Check performance is something complicated, but a solaris haves a bunch of nice tools who can help you to troubleshoot this, just like iostat -exnDl st1, where st1 is the tape drive address, with this you can see at OS level what the speed of the drive is between OS and drive, if robtest takes too long to respond check the library firmware version, probably the issue is with the library and not with NBU or Solaris.