Priority

You have to be root or the owner of a process to change its priority.

Reasons to Change a Process's Priority
If a high priority task has a flaw in it, a brute force attack will have more cycles to work with. Because of this, a low priority task is marginally more secure. However, the difference is so slight as to be not worth mentioning.

More importantly, a background task, like updatedb or emerge, can bring your system to a complete crawl. You'll want to make unimportant background tasks be of very low priority, or niced very high.

Files
Administrators can use /etc/security/limits.conf to make a certain user's default priority be higher or lower.

utilities
nice -20 is highest priority, 19 is lowest priority. renice top  r or n is the renice command,