Why qt execution is so slow,this is a test

[newgod]

Testing machine:
win7
8G
I7 2600

qt4.7.4 php5.3.8 python 2.6

run with qt

QTime t;
    t.start();
    QString aa="";
    for(int i=0;i<=1000000;i++){
       aa += "1";
     }
     printf("used: %d ms", t.elapsed());

QTime t;
    t.start();
    QString aa="";
    for(int i=0;i<=1000000;i++){
       aa += "1";
     }
     printf("used: %d ms", t.elapsed());

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debug :240ms
release:138ms


run with php

$time = new runtime;
    $time->start();
    $str = "";
    for ($i = 0; $i <= 1000000; $i++) {
        $str.="1";
    }
    $time->stop();
    echo "used:" . $time->spent() . "ms";

$time = new runtime;
    $time->start();
    $str = "";
    for ($i = 0; $i <= 1000000; $i++) {
        $str.="1";
    }
    $time->stop();
    echo "used:" . $time->spent() . "ms";

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used time :92.9ms


run with python:

import os,sys
    from time import clock as now
    start = now()
    str = ""
    for i in range(0,1000000):
        str += "i"
    finish = now()
    print (finish-start)

import os,sys
    from time import clock as now
    start = now()
    str = ""
    for i in range(0,1000000):
        str += "i"
    finish = now()
    print (finish-start)

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used time:210ms

run with qt c++:

QTime t;
        t.start();
        char *pStr = new char[1000001];
        for(int i=0;i<=1000000;i++){
            pStr[i] = '1';
        }
        delete pStr;
        printf("used: %d ms",t.elapsed());

QTime t;
        t.start();
        char *pStr = new char[1000001];
        for(int i=0;i<=1000000;i++){
            pStr[i] = '1';
        }
        delete pStr;
        printf("used: %d ms",t.elapsed());

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debug,3ms
release,1ms

not just this one test,every test just faster to python,there is no php half of the fast
is this my qt configuration's problem?
some one answer me,please