多线程Java不会加速

I have implemented a simple parallel merge sort algorithm in Java. This cuts the array into equal sections and passes them to be sorted independently by each thread. After the array segments are sorted, they are merged by a single thread. Because there are no shared resources, so no synchronization is used when the sub lists are sorted. The last thread which merges the result array though waits for the other threads to complete.I have implemented a simple parallel merge sort