Journal file is created with FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH attributes, but FILE_FLAG_WRITE_THROUGH is not needed here and even worse it flushes NTFS metadata on every write operation. Removing it doesn't affect durability (FILE_FLAG_NO_BUFFERING does it's job), but greatly improves performance and reduces disk load.

You can read detailed explanation here http://dmitry-naumov.blogspot.ru/2013/05/mongodb-vs-ms-sql-how-to-write-to.html