It has to do with JavaScript's "scope chain" method of looking up symbol bindings. If something isn't in the current scope, it has to incrementally check each "scope" (such a bastardization..) for the binding it's looking for. If it works its way back up to the global (window) object's "scope" and doesn't find it, it gives up and returns undefined.

"with", try/catch, etc. all introduce another level of scope, which means anything you reference that isn't in that scope directly takes more time to look up. Yeah, it's pretty retarded.