QT 5.7 Serial Port reads very slow

I am very new to programming and I am teaching myself. I wrote a application to poll a number of requests from a control unit . I basically continuously send various read commands to the Control unit and read back the response .My program works and i successfully send commands and receive answers . But the reading is very slow ( i have a 100 ms timeout in my code to ensure i get the complete reply ) I have a program for the same control unit that was written by a professional coder in C++ , in his program he polls every 30 ms and i always receive the complete answer in that time frame . I have the same settings 57K baud 8 bits 1 stop bit and no parity . However my QT code takes almost 100 ms to receive the answer. In my code i read the first 2 bytes ( first byte is the message identifier and second byte is the remainder of the message length) then i read in a loop until the total message length is equal to the message length byte +1 (the +1 is there to include the first byte ) . I am a wits end as to why my code is so slow in QT when i know its know the Hardware that's the limiting factor . The requests are always 3 bytes and the reply varies from 3 to 61 bytes . Please help me to point me to my error. If i remove the timeout i always have short reads . So far i also tried read(all) but with the same result . Below is the extract from my code where i read the response . The full code is at https://github.com/MarkusIppy/PowerTune

//Error handling 
QTime startTime = QTime::currentTime(); 
int timeOut = 100; // timeout in milisec. 
QByteArray recvData = m_serialport->read(2);  // reading first two bytes of received message to determine lenght of ecpected message 
int msgLen = recvData[1]; //Total message Lenght excluding the first byte 
while ( recvData.size() <= (msgLen+1) ) 
{ 
    if ( startTime.msecsTo(QTime::currentTime()) > timeOut ) break; 
    recvData += m_serialport->read(msgLen+1-recvData.size()); 
} 

if (msgLen +1 == recvData.length())  //if the received data lenght equals the message lenght from lenght byte + identifier byte (correct message lenght received ) 
{ 
    qDebug() << "Received data OK"<<msgLen +1; 
    if(requestIndex <= 61){requestIndex++;} 
    else{requestIndex = 58;} 
    readData(recvData); 
} 
else   //if the lenght of the received message does not correspond with the expected lenght repeat the request 
{ 
    qDebug() << "Received data lenght NIO"; 
    readData(recvData); 
    qDebug() << "Request Message again"<< requestIndex; 
}

I am sorry, I don't have enough time to go through your project and from the code you've provided I cannot be 100% sure what the cause is. My best guess though is that the problem in this case is that you wait explicitly for the data to be received and the events processing is somehow delayed or does not take place at all.

Anyway, here you have a couple of suggestions:

1. Use QTimer for timeouts instead of QTime.
2. Learn about the Qt5's signals and slots and use them to read from the serial port asynchronously.

I use the QSerialPort by connecting its bytesWritten(qint64 bytes) and readyRead() signals to slots of my program, let's say on_bytesWritten(qint64 bytes) and on_readyRead(). Then I send request to the target device and in the on_readyRead() slot I process the result. With each send command I start a QTimer with its timeout() signal connected to a on_timeout() slot of my application. This way I could monitor whether the device responds in time or not, as well as to have the data as soon as it comes. You may also use the errorOccurred(QSerialPort::SerialPortError error) signal of the QSerialPort to check if there is a problem with the transmission.