My experience is that Geektool can significantly affect performance on My Macbook Pro (early 2011 running Yosemite).
I've noticed that both Fans start to max out at 6000+ RPM and applications become sluggish. Geektool itself does not appear to be hogging CPU or memory but something related to it is. It's not the geeklets as they are only echoing out some text and running
df -h every 5 seconds.
One thing I've noticed is that the
WindowServer process seems to start eating a lot of cpu while Geektool is running. My hunch is that this is the problem - I use my Mac almost exclusively with a large external monitor attached.
So if you do experience performance lag, it may be the individual geeklets if they are doing anything intensive or some combination of Geektool + other applications which need to draw to the screen a lot. Try a search on this site for WindowServer too - there seem to be other people seeing issues with WindowServer causing performance problems on Yosemite.