Yeah, debug will butcher your framerate on the inspect tab, especially if you have a lot of arrays or objects.
One thing about cpu usage: the best thing to do is to export your project, either as html or as node-webkit. Sometimes your cpu will go down drastically on export, sometimes it won't, but it's best to see what happens. Be sure to have 'minify' checked.
If you are still using around 30%, something is almost certainly wrong.
Best way to figure it out: fire up debug and go to the 'profile' tab. See how much cpu is being used. If you have any groups, you can see aprox how much cpu is being used by each group. This is another reason why it's a great idea to break your logic up into groups and sub-groups; makes it easy to spot what events are lagging you down.
Also, check for extra behaviors on objects that you don't need, or loop logic going awry.