The wait times are due to all kinds of things.
In some cases, the delays don't have to do with all of the requested information. Some requests are prickly because they involve information belonging to third parties such as companies, which are, themselves, subject to an access to information regime.
A third party might be an airplane manufacturer or a food processor. Third parties tend to ask for more time to process the request. They may also take the matter to court or refuse to disclose commercial information. Third parties often cite the exemptions prescribed in section 20 of the act. Those tend to be the cases that drag on.