Welcome
Problem
Applications are cached query. Some queries are common and relatively long time. When the cache expires, they get to the base. In the same time many customers in cache misses, and asks the same query directly to the database.
During this time the database gets a big load, and performs the same query to multiple clients in several threads simultaneously causing a decrease database performance.

Solution
Useful to a flag (perhaps might get for queries with the flag slaveOK) which allowed the connection to the server queue which performed the same query to the query execution time. Data from this question were to be returned to all customers.

Place of Implementation
Mongos sharding or mongod for the replica set