In complex flows where the flow splits into several paths the FlowMessage needs to be cloned. When having large and complex structures in the FlowMessage this was a bottleneck. The internal handling of cloning was highly improved in 2.5.6
Each module has it's own queue for messages waiting to be processed. The default size for this queue was previously unbounded. This could cause high memory usage if the flow received more messages than it could handle for a long period of time.
To avoid this and instead use back pressure the default queue size will now be 100 to avoid high memory usage and high volumes of messages in queue.
The queue settings can still be changed, but the default behavior was changed in 2.5.6.