In some scenarios a file may be uploaded to GridFS in multiple steps. At each step an instance of MongoGridFSStream will be opened, some of the data will be written, and the stream will be closed. Currently the MD5 hash is computed every time the stream is closed. A significant performance improvement can be achieved if the calling code can turn off the computation of the MD5 hash for the intermediate steps.

See pull request: https://github.com/mongodb/mongo-csharp-driver/pull/57