In Process Viewer, I found iPhoto2 was using 20 to 35% of the CPU on my 1GHz PowerBook, even when hidden in the background! This was after going through and editing a few hundred JPEGs and then emptying out the library from within iPhoto. So it was just sitting there, hidden and empty.
After quitting and restarting iPhoto then hiding it, it maxes out at under 2% in Process Viewer. So it seems it will continue to leech resources after activity, but not if started up and then hidden and left dormant.
[robg adds: Somewhat vaguely related to this hint's topic, in testing this hint (I wasn't able to replicate the issue described above, but I'm also not able to exactly duplicate the conditions as I won't risk deleting my library for the sake of a test), I saw some strange figures out of 'top' (the Terminal's version of Process Viewer) for iPhoto2. After launching and not doing anything, CPU usage hovered around 40% to 50% (on a G4/733), just sitting in Organize mode in the background. It didn't make any difference if the window was minimized or not; it just sucked CPU time. But when I clicked into Edit mode, CPU usage dropped below 20%. If I clicked back into Organize mode, CPU usage shot up to 50% again.
Thinking it had something to do with my Organize view mode (black background, no shadows, border), I opened up the prefs and turned off borders. CPU usage dropped down under 20% again. But then I turned the borders on again, and CPU usage remained below 20%! So it clearly wasn't view options that were causing the problem.
So I quit and restarted iPhoto, and let it sit in Organize mode for several minutes, all the while churning away at 50%+ of my CPU (in the background!). I thought maybe iPhoto was doing something to the 2,000+ images in my library each time it launched (and I just happened to time it right when I was switching view settings in the first test). It appears this theory is correct, as CPU usage eventually dropped under 10% (and stayed there) without my changing any settings. Does anyone know what iPhoto might be doing for those first two or three minutes after it launches??]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030308035121230