Just started testing ST3 and have found that it occasionally spikes my CPU to 100%. The spike usually lasts for about five minutes. Just now, I changed the syntax highlighting of a 400-line SASS file to CSS and this seems to have caused the spike.
I don't have any plugins installed. Let me know if I can provide more details.
UPDATE: This is on OS X 10.8.2. First noticed on earlier builds but I'm currently running build 3012.