note: mongodb 4.0 is going to introduce Multi-document Transactions (see https://docs.mongodb.com/manual/upcoming/). This will make request serialization useless since its goal is best addressed via transactions.
in order to avoid ghost writes that might happen when concurrent write request are received we want to add the ?s query parameter (s=serialize). this qparam will serialize the concurrent requests thus avoiding this issue.
being serialized and not performed concurrently, the write operations will take more to complete, i.e. decreased performances.