Previously before, I didn't touch apps like GeekTool because I was afraid that it would slow down my system performance severely. (MacBook Pro 15" C2D 2.4ghz Late 2008) However, I have decided to use it as a clock on my wallpaper.
How much CPU % would GeekTool use? I only have the measly 2 hour battery for the MacBook Pro, so conserving as much CPU cycles as possible would be my aim.
Best Answer
GeekTool itself uses minimal CPU as shown by the other answers.
However, GeekTool really just facilitates running other scripts. It's the frequency and impact of those scripts on your system you want to worry about. By tuning them in their runtime and CPU load you can have GeekTool present useful information with minimal system impact.
I would focus less on GeekTool itself, and more on what you're running with it.
eg. on the clock example above, have it update the time once a minute, rather than every second.
Weather geeklets can be run once every 15-30 mins as you feel is appropriate.