int getPID(QString processName)
{
    QDirIterator iterator("/proc");
    QString s;
    QString name;
    QFile f;
    while (iterator.hasNext())
    {
        s = iterator.next();
        f.setFileName(s + "/comm");
        if (f.open(QIODevice::ReadOnly))
        {
            name = f.readLine();
            f.close();
            if (name.contains(processName))
            {
                return s.split("/")[2].toInt();
            }
        }
    }
    return -1;
}

kemi242@linux-93id:~/getpid/bin> time ./getpid_ps

real    1m21.628s
user    0m29.981s
sys     0m50.390s

kemi242@linux-93id:~/getpid/bin> time ./getpid_qt

real    0m13.016s
user    0m5.858s
sys     0m7.153s
kemi242@linux-93id:~/getpid/bin> time ./getpid_tch

real    0m2.815s
user    0m0.354s
sys     0m2.459s

@TCH:
Dehogynem, hiszen a te overloadolt operátorod is ugyanazt csinálja: meghívja a konverter metódust, de azt te is csinálhatod: cout << s1.toStdString() << s2.toStdString() << s3.toStdString();