I know you are asking about Java, but I use mutable vs immutable all the time in objective-c. There is a immutable array NSArray, and a mutable array NSMutableArray. These are two different classes written specifically in an optimized way to handle the exact use. If I need to create an array and never alter it's contents, I'd use NSArray, which is a smaller object and is much faster at what it does, compared to a mutable array.

So if you create a Person object that is immutable then you only need a constructor and getters, thus the object will be smaller and use less memory, which in turn will make your program actually faster. If you need to change the object after creation then a mutable Person object will be better so that it can change the values instead of creating a new object.

So: depending on what you plan on doing with the object, choosing mutable vs immutable can make a huge difference, when it comes to performance.