MongoDB count and get limited data
As per MongoDB BLOG Here In certain situations, count can be avoided for better performance Counting things can be hard on a database. The obvious things to count, number of items in a collection or index, are easy to keep track of, but as soon as you start trying to count records based on a query, things get slower and slower. This is because the database has to do more work scanning practically every item in the database to see if it matches.

MongoDB does have an optimisation for a fast count where all the queried fields are indexed and the query is based around equivalence, but only in that circumstance.