Avoid repeated fieldname length calculation in BSONObjBuilder's appendElementsUnique(). The loops that iterates over the object with the to-be-added fields calls fieldName() on each BSONElement. This only returns a const char* pointer without the field name length. This leads to the creation of an std::string temporary and the repeated calculation of the field name length. Both can be avoided by calling fieldNameStringData() on the underlying BSONElement.