Background check includes criminal (Police Clearance) and security (National Security) checks. All adult applicants for immigration to Canada and ALL their dependents, whether accompanying or not, are required to contact the police or relevant authority of each country where they have lived for six months or longer in the last ten years and obtain the necessary police certificate or clearance stating that they do not have criminal convictions.
A National Security check procedures identify persons seeking admission who are, or have been, involved in espionage, subversion, or terrorism.
These checks involve CSIS, RCMP, and local security authorities in the countries lived.
They take a while as it is not just one entity doing the work, but one entitity communicating with others [internationally, and that can take a while].