Registering global JsonConverter is very costly, The best thing is to specify them as a contract (see ElasticContractResolver) This however prevents a way to give a JsonConverter state which for some calls is needed i.e: A multiget and multisearch need access to the descriptor that describes what types are used. When NEST knows it has to piggyback this it has to pass serialization state it will create a new serializersettings object with a new contract resolver which holds this state. Its ugly but it does boost massive performance gains.