sara ji u have asked a good question and infact hardly anybody wud be to answer ur question in an appropriate way.
but as long as i think that there are twelive subjects in the css. and u have to get mastery over all subjects and another thing is that english composition, essay and EDS are bit difficult and low scoring subjects. moreover, the trend of marking goes on changing. their is no subject in the css which always give same kind of marks. suppose it was considered that sociology gives lot of marks.but this hypothesis proves false in 2008 exams. so it is very defficult to think that which subject gives more marks and which gives less.
therefore, sara jii css needs a lot of hard word and very wide reading. obviouly it takes a lot of time.
so only the few candidates are capable to work hard and get through the css.
hopefully some of my personal views wud be correct.
however, many other can throw light on ur question in a better way.