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If the dump process is doing a lot of I/O without sending data, then the pv solution will not work.

You can try to set the nice command (like ionice) in the xinet or inetd configuration file.
...

Another possibility is to slow down the transfer to the server:

In the dumptype:
encrypt client
client-encrypt "/usr/bin/pv-300k"

$cat /usr/bin/pv-300k
#!/bin/bash
exec /usr/bin/pv...

If it's a disk I/O problem, try the solution from https://forums.zmanda.com/showthread.php?2543-Using-ionice-to-reduce-disk-I-O-impact