UI hangs when either QThread or QTimer invoked

[saman_artorious]

My program continuously reads from serial port. Firstly, I used QTimer and set the interval to 300 mlscnd, this makes UI interaction very slow. I though of converting QTimer to QThread, as QTimer runs in the UI Thread, so, I added QThread class and used msleep() inside the while() loop to control the trigger.
Unfortunately, the program is still slow, I can clearly understand this when I enter digits in the UI. My last guess of where the speed reduction might have come from is emit, as you see below:

void PlcTimer::run(){
    while(1)
    {
        qDebug() << "updateJack()";
 
        const char str[]={UPDATE_m};
    #if DEBUG
        QByteArray  built(str, 3);
        qDebug() << built.toHex();
    #endif
        emit m_sendToRS((char* )str, 3);
        this->msleep(500);
        //sleep(300);
    }
}

void PlcTimer::run(){
    while(1)
    {
        qDebug() << "updateJack()";

        const char str[]={UPDATE_m};
    #if DEBUG
        QByteArray  built(str, 3);
        qDebug() << built.toHex();
    #endif
        emit m_sendToRS((char* )str, 3);
        this->msleep(500);
        //sleep(300);
    }
}

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emit is a UI object, so whatever it is I am suspicious of its interconnection over the UI. do you have any idea how I can resolve the low speed?