When we merged in #1849 , we did it knowing that there were still improvements that could be made to the code. In particular, there are 2 outstanding issues with that code that we know about:

1. There is a TODO in the code at

   rclcpp/rclcpp/include/rclcpp/publisher.hpp

   Lines 333 to 343 in 6b321ed

   	if (inter_process_publish_needed) {
   	ROSMessageType ros_msg;
   	// TODO(clalancette): This is unnecessarily doing an additional conversion
   	// that may have already been done in do_intra_process_publish_and_return_shared().
   	// We should just reuse that effort.
   	rclcpp::TypeAdapter<MessageT>::convert_to_ros_message(*msg, ros_msg);
   	this->do_intra_process_publish(std::move(msg));
   	this->do_inter_process_publish(ros_msg);
   	} else {
   	this->do_intra_process_publish(std::move(msg));
   	}

   . In particular, if we are calling do_intra_process_publish, then it may be the case that as part of that call we are doing a conversion from PublishType -> ROSMessageType so that we can store the ROSMessageType in one or more of the subscribers. But if we did that in do_intra_process_publish, we are unnecessarily doing it again in the publish call directly. The answer is probably to make a new overload of do_intra_process_publish_and_return_shared which always returns the ROSMessageType, and then use that directly when doing the inter-process publish. This probably requires plumbing all the way down to the IntraProcessManager layer.
2. There's a discussion in Add non transform capabilities for intra-process #1849 (comment) about simplifying the number of cases we need to handle in AnySubscriptionCallback. Currently we allow the signature of the Subscription callback to be a different (though convertible) type from the template type we passed to create_subscription. However, if we force the two of them to be the same, then we can get rid of several case statements when dispatching data. That would help a lot in maintenance since there are a lot of cases handled during dispatch currently.