What are soft errors and why are they slowing down my backup (4046984)

Soft errors are correctable write errors generated when writing to tape. All tapes will give soft errors because tape is a magnetic media that will almost always have some bad segments on it.

So If the data segment has a write error, the tape will be advanced to another section of tape and the same data segment is written again. This is a Soft Write Error. The repeated write action will take more time and more tape space.

To understand how many soft errors are acceptable per tape you need to speak to the hardware manufacturer or hardware maintainer.