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

I would like to note that ReaderWriterLockSlim is much slower to acquire than Monitors (lock) and should only be used if readers spend considerable time (15ms or greater IIRC) inside the lock. If reader does a quick job and contention isn't 100% for each of the cores you have, Monitor is the way to go (well, unless you're heavy on async side of the story, but that's unusual in most cases).