The code that is generated when one specifies a type with serialisation type of 'any' looks like this:

        BSONArrayBuilder arrayBuilder(builder->subarrayStart(kWriteErrorsFieldName));
        for (const auto& item : _writeErrors.get()) {
            const BSONObj localObject = item.serialize();
            arrayBuilder.append(localObject);
        }

This means that the serialiser method returns a free-standing BSONObject, which is then copied into the main builder.

It would be more efficient if the serialiser generated looked like this in order to avoid the data copy:

        BSONArrayBuilder arrayBuilder(builder->subarrayStart(kWriteErrorsFieldName));
        for (const auto& item : _writeErrors.get()) {
            item.serialize(arrayBuilder.subObjectStart(localObject));
        }

This would require all places that use serialisation type of any currently to be changed so that their serialise method takes a BSONObjBuilder instead of returning BSONObj.