Generally speaking if a file is prepared fully, with all documents included prior to lodgment it is easier for immigration to make a quick decision when your file is picked up to be assessed.
Usually there is prescribed waiting period for each type of visa because of either the current number of such cases pending, Country were the application is lodged and qualify of your documents provided.
Besides the above every case is different and could have other issues, like health, character, lack of evidentiary documents, interviews etc. that can cause delays.
