There's more to lock than just the write, some of this stuff will be lots better in future Mongo releases, but at the moment doing a safe write means a lock is

There's more to lock than just the write, some of this stuff will be lots better in future Mongo releases, but at the moment doing a safe write means a lock is held longer when there's network latency of any kind. Local updates are shockingly faster than even local network updates. Honestly, the entire point of Mongo isn't that it's fast, it's that it's fast enough and can scale a reasonably flexible schema through the life of an app.