If you seriously have not cleaned your system then the cap would be a likely suspect, especialy the ones with the screen around them. It's usually the first part of the system to clog up enough to make a noticeable difference providing you have one.
If you haven't cleaned and if you have a cap and if it's clogged I would just clean the whole system while your at it and plan on doing it annually.
If you haven't cleaned and if you have a cap and if it's clogged I would just clean the whole system while your at it and plan on doing it annually.