I would check your Read Speeds from the Source Disk Library.
I would test your Tape Drive Throughput out to to make sure your Writes are as fast you would expect.
The DDB would not really come into play for your Tape copy.
Yes SAN attched Index Cache is not reccommened and could be the bottle neck ( since it is easy to move) can you move it back to local SSD to see if this majes an improvement? If it does not then check the other items I noted.