Filesystems, Disks and Memory

I've starting playing with a tape drive for the first time. Its a new one, an IBM Ultrium LTO 4. After initially copying a file to the tape with tar cvf /dev/st0 /root/dummy, all subsequent files appended take an increasing amount of time with tar rvf /dev/st0 /root/dummyX. Is it normal for each appended file to take longer than the previous? Also, anyone have an idea of an optimal block size?

no it shouldn't take long either. you can omit running in verbose mode. by the way, whats the system you're using and what OS?

ANd yes, I forgot..Please check for the SCSI termination.
If possible reseat the terminator. If you doubt O.S. then pls use other command e.g. cpio and try to get the speed.