With Java 8 and its Stream API, we have seen a brand new way of processing collections of data. One of the most powerful features of the Java8 Streams is its possibility of paralellize the operations pipeline. But, in some cases, there has been several developers which has found that not always the parallel stream works better than the serial stream (or even the traditional for or foreach loops to iterate over a collection). In this talk we will take a look at the Streams performance and we will present some of the elements that could affect the performance. We will also present the mechanisms under the Parallel Stream in order to know better the way it works and to try to understand what could affect to the performance of a parallel stream