Why is the machine I'm on so slow?

You may have a process that is taking up a lot of CPU usage. To see if that’s the case, type ps at the command prompt. You will see something like the following:

      PID TTY        TIME CMD
    13391 pts/3  00:00:00 tcsh
    13470 pts/3  00:00:01 emacs
    13974 pts/3  00:00:00 ps

To get rid of a process, type kill -1 PID where PID is the number in the PID column. For instance, to get rid of emacs in the above example, the user would type kill -1 13470. This should clean the process up nicely. If after a short while, ps still shows that process, type kill -9 13470.

On Linux you can also use the graphical utility ksysguard, and on macOS you can check the CPU tab of the Activity Monitor application.