If your application is I/O bound, which from your description it appears to be, then nice won't do much for you. The OS is already handing cycles to other processes while waiting for I/O opperations. Although the high CPU utilization would point to some serious CPU cycles being spent somewhere. I can't tell from your description where that would be. A better indication of system load can be had with the uptime command.

You don't say what unix you're running on. It makes a difference as System V multitasks different than BSD.