Hello, I am writing an Imap client but I need Your help to improve my code, I have tried to solve it, but I don't know how. I guess it is some problem with too big data stored in QByteArray

As first my program generates very hight CPU usage when I try download using Imap, it is 100% of my Core i7. I am sure it is some bug in download function because as normal does not exceed 10%
Second problem I found is that download speed slows down about 10kb/s and finish on few bytes/s making my client unusable, (can't download bigger file than 3mb)


Imap.cpp (download function)
Qt Code: 
  1. void Imap::downloadFile(int message_id, QString filename){
  2.     m_logger->Debug("IMAP - downloading: " + this->m_login);
  3.     this->login();
  4.     sendCommand("a03 EXAMINE INBOX", "a03");
  5.     m_dowloadFile = true;
  6.     qint64 size = 0;
  7.     for(int i = 0; i < m_mailData.count(); i++){
  8.         if(m_mailData[i].messageId == message_id){
  9.             size = m_mailData[i].size;
  10.         }
  11.     }
  12.     this->getProgressListener()->startTimer();
  13.     this->getProgressListener()->setTotal(size);
  14.     m_file = new QFile(filename);
  15.     if(m_file->open(QIODevice::WriteOnly)) {
  16.         sendCommand("a09" + QString::number(message_id) + " fetch " + QString::number(message_id) + " BODY.PEEK[2]", "a09" + QString::number(message_id));
  17.         m_file->close();
  18.     } else {
  19.         m_statusCode = Mailbox::FileWriteError;
  20.     }
  21.     this->disconectFromHost();
  22.     delete m_file;
  23.     m_file = NULL;
  24.     m_dowloadFile = false;
  25. }
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Part of SocketsBase.cpp
Qt Code: 
  1. ocketsBase::SocketsBase(QString server, int port, bool ssl_mode, EConnectionType type, QObject *parent) :
  2.     QObject(parent)
  3. {
  4.     m_server = server;
  5.     m_port = port;
  6.     m_SSLMode = ssl_mode;
  7.     m_logger = Logger::getInstance();
  8.     m_connectionType = type;
  9.     m_file = NULL;
  10.     m_dowloadFile = false;
  11.     m_finished = false;
  12.     m_running = false;
  13.     m_timer = new QTimer;
  14.     m_Sockets = (m_SSLMode) ? new QSslSocket(this) : new QTcpSocket(this);
  15.     connect(m_Sockets, &QAbstractSocket::readChannelFinished, [=]()
  16.     {
  17.         emit finished();
  18.     });
  19.  
  20.     connect(m_Sockets, SIGNAL(readyRead()), this, SLOT(ready_read()), Qt::DirectConnection);
  21.     connect(m_Sockets, SIGNAL(connected()), this, SLOT(connected()));
  22.     connect(m_Sockets, SIGNAL(error(QAbstractSocket::SocketError)), this,SLOT(errorReceived(QAbstractSocket::SocketError)));
  23.     connect(m_Sockets, SIGNAL(stateChanged(QAbstractSocket::SocketState)), this, SLOT(stateChanged(QAbstractSocket::SocketState)));
  24.     m_success = true;
  25. }
  26.  
  27. void SocketsBase::ready_read()
  28. {
  29.    m_finished = false;
  30.    connect(m_timer, &QTimer::timeout, [=]() {
  31.         emit timeOutReached();
  32.     });
  33.     m_timer->start(240000);
  34.  
  35.  
  36.     if(m_dowloadFile)
  37.     {
  38.        ProgressListener::getInstance(m_login)->updateProgress(ProgressListener::getInstance(m_login)->getCurrent() + m_Sockets->bytesAvailable());
  39.        m_logger->Debug(trUtf8("Bytes available %s"), qPrintable(QString::number(m_Sockets->bytesAvailable())));
  40.     }
  41.  
  42.     while(!m_Sockets->atEnd()){
  43.        if(m_Sockets->bytesAvailable() > 4096){
  44.           m_dataBuffer.append(m_Sockets->read(4096));
  45.        }else{
  46.           m_dataBuffer.append(m_Sockets->readAll());
  47.        }
  48.     }
  49.     m_Sockets->flush();
  50.     m_logger->Error(QString::number(m_dataBuffer.size()));
  51.  
  52.  
  53.     QRegExp re1(m_currentCode);
  54.     QRegExp re(".[\r][?\n]?a[0-9]+ OK Success.*");
  55.     QRegExp re2(".[\r]?[\n]?a[0-9]+ OK [fetchFETCH]+.*");
  56.  
  57.     if(((re1.indexIn(m_dataBuffer) != -1 && !m_dowloadFile) || (m_dowloadFile && re.indexIn(m_dataBuffer) != -1) )|| re2.indexIn(m_dataBuffer) != -1 || m_currentCode == "-1" || m_connectionType == POP3)
  58.     {
  59.         if(m_dowloadFile)
  60.         {
  61.             if(m_file)
  62.             {
  63.                 if(!m_file->isOpen())
  64.                     if(!m_file->open(QIODevice::WriteOnly))
  65.                     {
  66.                         m_logger->Error(trUtf8("Couldn't open file %s to write"), qPrintable(m_file->fileName()));
  67.                     }
  68.  
  69.                       QTextStream out(m_file);
  70.                       out << m_dataBuffer;
  71.                   //...
  72.               }
  73.         }
  74.         m_timer->stop();
  75.         emit finished();
  76.     }
  77.     //
  78.     //...
  79.     //...
  80.     //
  81.       else if(m_dataBuffer.contains("NULL") || m_dataBuffer.contains("null")){
  82.         m_logger->Error("Buffer is null !");
  83.         m_dataBuffer.clear();
  84.         emit finished();
  85.      }
  86. }
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m_dataBuffer is QByteArray

Any suggestions ?