nice allows you to change the priority of a utility that is not running. You must give the -n flag, like so:
samizdata# nice -n 20 nmap -sS -sU -O localhost
Using the -n flag allows you to set the priority that the process will run at, using a numeric value from -20 to +20; the lower the number, the higher the priority.
Note that this is not for already running processes, for that, see renice.
See also renice