In the Advanced Options section of Preferences, there is a slider that goes from "Fast CPU" to "Slow CPU". What does that do? Is it used to indicate that your computer is fast or slow, or is it used to indicate what kind of response other programs get when ktorrent is running?

This only has effect when you have speed limits. With speed limits we need to sleep now and then, this slider controls the sleep interval. Setting it to fast CPU could result in higher CPU usage (depends on the speed of your processor), setting it to slow CPU can hurt high speed download speeds, but will reduce CPU usage.

If you are experiencing high CPU usage, set it to slow CPU, normally it shouldn't hurt speeds on your average internet connection.