When I started using ECB (http://ecb.sf.net) with the Ubuntu Emacs 22 snapshot (both X and GTK variants), the CPU usage soon shot up to 100% and stayed there.
The solution, posted at http://sourceforge.net/mailarchive/forum.php?thread_id=30578186&forum_id=1620, was to update the installed version of semantic-idle.el because of how Emacs 22 uses timers.
Unfortunately, I couldn’t find an easy way to persuade Emacs to load a version of CEDET built from CVS instead of from the Ubuntu Debian packages.