Not only multithreading

FP is not an optimization

FP gets you the better code

Let's start simple

public int length(String s) {
  return s.size;
}
public bool isEven(int i) {
  return i % 2 == 0;
}
public bool isStringLenghtEven(String s) {
  int i = length(s);
  return isEven(i);
}

public Function<String, Boolean> compose(f1 Function<String, Integer>,
                                         f2 Function<Integer, Boolean>) {
  return s -> {
    Integer i = f1.call(s);
    f2.call(i);
  }
}

public Function<A, C> compose(f1 Function<A, B>,
                              f2 Function<B, C>) {
  return s -> {
    B i = f1.call(s);
    f2.call(i);
  }
}