It is tempting to just set up all the streams parallelly and not think about it again. Unfortunately, parallelism does not always provides an advantage. In fact, it incurs an overhead because of the worker threads' coordination. Besides, some stream sources are sequential in nature and some operations may share the same (synchronized) resource. Even worse, the usage of a stateful operation can lead to an unpredictable result. It is not that you cannot use a stateful operation for a parallel stream; it requires careful planning and clear understanding of the state management.