The nice command lets you run a command at a priority lower than the command's normal priority.  The nice value can range from 0 to 39, with 39 being the lowest priority.  For example, if a command normally runs at a priority of 20, specifying an increment of 5 runs the command at a lower priority, 25, and the command runs slower. 

A standard user usually has a nice level of 20 so you are able to reduce the nice level of a user from 20 to 39 (0 to -19).  Edit the /etc/passwd file entry for the user, and run the users shell program at a lower nice level.