Not for atomic writes, which is the important bit here. When dealing with concurrency the choice to make us will you allow tears when writing i.e. thread A writes aa and thread B writes bb. Will you allow the option of seeing ab or ba or only aa/bb. This is the thing that costs performance. Plus do you allow null which makes it harder too.

It's about the semantic guarantees because you don't want to introduce a parallel programming bug just because you added a field to a type. Tearability is opt in and if you are serious about performance you will make the correct choice.

It's mostly a performance question in order to adhere to Java's memory model.