多线程的效率一定快吗？

//必须是final，否则会报错
    private static final long count = 100000000;
    
    @Test
    public void testEfficiency() throws Exception {
        
        serial();         //串行
        concurrence();         //并行
    }

    private void concurrence() throws InterruptedException {
        long start = System.currentTimeMillis();
        Thread t = new Thread(new Runnable() {
            
            @Override
            public void run() {
                int a = 0;
                for(long i =0;i<count;i++){
                    a ++;
                    a ++;
                    a ++;
                }
                
            }
        });
        
        t.start();
        
        int b = 0;
        for(long i = 0;i<count;i++){
            b --;
            b --;
            b --;
        }
        t.join();
        
        long end = System.currentTimeMillis();
        System.out.println("并行执行时长："+(end - start));
        
    }

    private void serial() {
        long start = System.currentTimeMillis();
        int a = 0;
        for(long i=0;i<count;i++){
            a ++;
        }
        int b = 0;
        for(long i=0;i<count;i++){
            b --;
        }
        long end = System.currentTimeMillis();
        System.out.println("串行执行时长："+(end - start));
    }