I wrote a lot about the performance metrics of the equals method and hash code in this article. There are many nuances that can lead to performance problems in those methods. The problem is that some of those things can be well hidden.
To summarize the core problem: the hashcode method is central to the java collection API. Specifically, with the performance of hash tables (specifically the Map interface hash table). The same is true with the equals method. If we have anything more complex than a string object or a primitive, the overhead can quickly grow.