why Qt5.5.1 qDebug so slowly?

Hello everybody.

i use Qt5.5.1 , I found debug so slowly.

code:
QTime time;
time.start();
for(int i = 0;i<1000;i++)
qDebug()<<" , hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello, hello";
qDebug()<<"time is : "<<time.elapsed()<<" ms"; //time is : 17453 ms

It takes 17453 ms from the qDebug() . but i use the same code in qt 5.3.1 , How to solve this ? :(