Synchronization in the ClassMap class is the number one performance issue in our performance tests - specifically, in BSON.applyDecodingHooks(Object)

The problem is that this lock is acquired repeatedly by any thread that is decoding elements, leading to a comb pattern of acquisitions and almost certainly not only guaranteeing contention but a moiré pattern of it leading to heavy switching overhead.

A better strategy would be a CopyOnWriteMap. We have an Apache licensed CopyOnWriteMap impl if required.