New key in EM_CONF array, ‘doNotLoadInFE’ can be set to “1” for extensions that are only used in backend and have no front end components. This speeds up the frontend generation process, since extensions not doing anything in the FE will not be loaded on FE calls.
More info:
RFC #11474: Store separate extlist for the frontend
http://bugs.typo3.org/view.php?id=11474