It means your case will not move forward until the security clearance is done. Your case is basically stuck until then. Did you receive a notice in the mail for a fingerprinting appointment? Perhaps you should inquire at USCIS about it. They should have sent out a notice if you needed fingerprinting such as if it has never been done before or if it's been a while since you last had them done - however, it wouldn't be the first time they either sent such a request to a wrong address, did not send it out at all or something somehow got lost. Once the security clearance is completed - provided you have no security issues such as a criminal background - depending on the type of AOS, it should take a few months only. 2-4 months is for problem-free marriage-based AOS (you did not mention if your AOS is based on this). Good luck!
This advice does not form an attorney-client relationship and is merely informative. It should not by itself be relied upon to address a legal concern.
that doesn't strike me as terribly long, but you can schedule an infopass at this stage if you're particularly concerned
This is not legal advice and no attorney-client relationship has been formed, now or in the future. This is just a casual opinion expressed about a hypothetical situation.
It means exactly that, that the case will not proceed until the security checks are completed. How long security checks take do not depend on the size of the office handling the matter.
The Court of Appeals for the Fifth Circuit has stated that "federal immigration laws are exceedingly complex"
You should retain an experienced immigration lawyer to review all the facts, advise you, and handle the case. You can find one through http://www.ailalawyer.com.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.