The presence of ObjectToObjectConverter in a DefaultConversionService causes the guard trying to register converters for ObjectIds in AbstractMongoConverter to not trigger the registration. This in turn caused ObjectId conversions to be executed via reflection instead of the straight forward method calls and thus a drop in performance for such operations.
We no unconditionally register the converters to make sure they really get applied.
Related tickets: SPR-13703.
c876155d34