Just to clarify, Background checks are too see if the information you entered is true.
Such as: Dependants, they check with the countries you lived in too see if you are not paying child support or have your name on any birth certificates, they cross reference marriages as well to see if you are legit and not marrying someone here then will do the dirty once you are landed. This protects the person sponsoring as they are taking on a financial responsibility by bringing you into the country for 3 years.
Security checks are handled by CSIS and this is the CIA of Canada, nobody is certain what is looked into but it can be assumed Criminal records, military records and again if you are associated with any gangs etc or have been in the past.
Lastly depending if you are from a high risk part of the world will determine the length of the checks sometimes. Also depends on how fast your government corresponds with Canadian Authorities is another.