MongoDB query execution time
Update based on MongoEngine usage in comments:

MongoEngine defers to pymongo for the connection pool setting. Hence, based on the pymongo FAQ:


  In versions before 2.6, the default max_pool_size was 10, and it did
  not actually bound the number of open connections; it only determined
  the number of connections that would be kept open when no longer in
  use.
  
  Starting with PyMongo 2.6, the size of the connection pool is capped
  at max_pool_size, which now defaults to 100. When a thread in your
  application begins an operation on MongoDB, if all other connections
  are in use and the pool has reached its maximum, the thread pauses,
  waiting for a connection to be returned to the pool by another thread.


Assuming you are using a version later than 2.6 you will have a default of 100, which theoretically should be enough.  However even if the responses are very quick you still have some overhead (connect, run query, return result, return connection to pool) with the pool in order to reach 100 concurrent users on the front end. Therefore, I would still look at increasing the max_pool_size parameter to see if that unblocks your load testing (say 150 to start with).