mongodb/mongo-csharp-driver — comment on commit 4c1b6d2a80cf1b9b87157b9d758ebe63d864b9ef
File:  line 0
By rstam:

I totally agree with you. We switched to ReaderWriterLockSlim in the BsonSerializer because once everything was finished lazily initializing the access pattern became 100% readonly, so we traded off the additional overhead of ReaderWriterLockSlim for increased concurrency.