Idle ROS nodes using >100% CPU

edit

i've built/written a couple of my own ROS nodes. They are connected like this:

create_data_node mqtt_bridge =>|broker|=> mqtt_bridge => processing_node
        master1/ pc1                        master2/pc2

create_data_node does just what you would expect - creating data and then publishing it onto the mqtt_bridge, which in return will publish them onto the broker. From there another mqtt_bridge (on another pc) received the data and published it to processing_node which in return does some processing. The workflow itself has to be that way, works out when data is created and is not part of this question.

Now: The moment i shut down create_data_node everything on master2/pc2 should enter idle mode, since no data is put in and no callback are called. Yet the CPU usage for this particular node is > 100% and furthermore rosout is using ~150% as well.

On processing_node i'm simply creating a ros::Subscriber and then using ros::spin(), just as follows (stripped example):

int main(int argc, char** argv) {
    ros::init(argc, argv, DEFAULT_NODENAME);
    ros::NodeHandle n;

    ros::Subscriber sub = n.subscribe<messages::testmessage>("topic", 1000, callback);

    ros::spin();
    return 0;
}

As far as i understand ros::spin() (in contrary to while(1) { ros::spinOnce();} should not do an idle wait.

So my question simply is - What causes this behaviour and what can i do to prevent this? Thanks in advance!

Edit: Reducing the subscriber queue to 100 resulted in 60% CPU and further reducing to 10 in 50% CPU usage.