Is there any way to set the priority of a process in Mac OS X?

From the command line (Terminal.app or whatever) use nice and renice, just like on other unixes.

Use nice when launching a process:

nice -n <priority> <command> <arguments to command>

The default priority is zero, positive values are "nicer" (that is lower priority) and negative values are "less nice" (higher priority). Looks like Mac OS runs from +10 to -10.

Use renice to change the priority of a process already running (from the renice man page on 10.5):

renice priority [[-p] pid ...] [[-g] pgrp ...] [[-u] user ...]  
renice -n increment [[-p] pid ...] [[-g] pgrp ...] [[-u] user ...]

The part you're interested in here is the pid bit. That is the process id for the job and you can find it using ps -u <your username> and looking for the process name, but I prefer top -o in this case, because the process you're interested in will be near the top.

Note: Without superuser privileges you can never increase a process's priority. For normal users, nice and renice are one way streets. And small changes in priority can have large effects on running time. So go easy on this until you understand it.

You can use the command:

renice -n # PID

Where:

• # is a number that should be larger than 0 (otherwise you will move your process to higher priority)
• PID is the process ID you can view by typing top on the terminal app (utilities/terminal.app)

If it is a system process or another user process you should type:

sudo renice -n 10 PID

It will ask you for your password (if you are sudoer). As for the number I would recommend 10 or 19 (even lower priority).

Note that this will change the priority not the CPU usage. If you aren't running other processes which require CPU or you have more than one CPU on your Mac (Core 2 Quad Core) the process might still use 100% of CPU.

renice 20 $(pgrep ImageOptim)

Or use the name of your program instead of ImageOptim