Hi, you asked for my CPU usage. I have a Mac G5 dual 2.5 Ghz, 1.5 GB RAM, ATI Radeon 9800 XT Pro (256 MB), PPC, OS X 10.4.8.
I tried to use some of the memory analyzing programs in the Developer suite, but I'm not too sure how they works, so I used the old 'top' method in Terminal.
In normal operation, without moving the mouse or doing anything that affects Focus, it's between 0.6% - 1.1% CPU (never less, never more than this).
If I switch from the current program to another program, Focus goes up to about 4.5% CPU, and stays there for about 1.0 seconds.
Maximum CPU usage value that I've seen so far: 10.3% CPU.
At the time of this test, I only had Safari, Mail and Terminal open. Although, I'm using a lot of window-less apps such as iCalViewer, Zooom, Relaunch, MacMinder, etc.
I don't mean to say that Focus is a complette memory hog, not by any chance. :) But if it could help in any way, these are my statistics. Good luck with further improvement and thanks a bundle, for a very nice application! (I definitely intend to keep using it!).