It's not a good idea to
nice a foreground job (Section
23.3). If the system gets busy, your terminal could "freeze"
waiting to get enough CPU time to do something. You may not even be able to
kill (Section 24.11) a nice'd job on a very busy system because the
CPU may never give the process enough CPU time to recognize the signal
waiting for it! And, of course, don't nice an interactive program like a text editor unless you
like to wait... :-)