No, this is not a post about electricity conservation (although, really, you should). Rather, it's about the open windows on your computer. The fact is, when you're using an old G4 tower you need to squeeze every inch of performance out of that processor and be wary of ways which sap said performance.
What I'm talking about here are animated progress bars, scrolling text, etc. By keeping application windows open while their tasks run in the background, you're robbing your processor of precious speed. For example, that progress bar in your browser's download window uses CPU cycles. Close the window while it downloads and you'll see a percentage of your processor freed up. Another example--in iTunes the scrolling text while playing a track, or especially playing a radio station, uses an excessive amount of CPU. Close the window as the track continues playing, and your processor load will return to practically nil.
There are other examples. When I have Transmission's window open, the application uses 7-8 percent of total CPU. When I close the window and it continues seeding and leeching in the background, the CPU load decreases to less than 1 percent. The two video encoders, Handbrake and ffmpegX, both have animated progress bars that use up about 10 percent of my processor. Close the window and the encoding speed magically increases 10 percent (no overclocking necessary)!
Oftentimes you can also minimize the window to dock rather than close it out altogether.
Now each of these instances by themselves won't make that much difference, but when you have an iTunes track playing while Transmission seeds in the background and your browser is downloading a file, it can all add up to something substantial and make your 500MHz G4 Screaming Sawtooth suddenly feel like an old Edsel. And we don't want to go back there.