Currently the rate is only output at the end of execution. Just test a smaller range -- you can edit SoBStatus.dat manually or give the range on the cmdline, as in
"proth_sieve.exe 50000000000000 50000024000000", in which case SoBStatus.dat is overwritten so be careful not to trash any work in progress.

As for a priority parameter, use the "nice -n20" (for linux) or "start /LOW" (windows) utilities for now. I'm too lazy to do that.

Mikael