@SGaist
I don't see why a temporary image would fare better than a buffer. I would have to read the file and re-fill my buffer if it fails, resulting in more time lost.

connect(m_pImageCapture, SIGNAL(imageCaptured(int,QImage)), this, SLOT(processCapturedImage(int,QImage)));

Then the slot:
void xxxxx::processCapturedImage(int requestId, const QImage& img)

You can extract from the QImage passed to the slot.

?

MainWindow.c:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    //qDebug() << "QCameraInfo: " << checkCameraAvailability();
    QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
    camera = new QCamera(cameras.first());
    imageCapture = new QCameraImageCapture(camera);

    connect(imageCapture,&QCameraImageCapture::imageAvailable,this,&MainWindow::testFunc);

    camView = new QCameraViewfinder();
    camera->setViewfinder(camView);
    camView->show();
    camera->start();

    ui->horizontalLayout->addWidget(camView);
    ui->horizontalLayout->addWidget(&testLabel);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::testFunc(int id, QVideoFrame buffer)
{
    qDebug() << id;
}

void MainWindow::on_SnapButton_clicked()
{
    imageCapture->setCaptureDestination(QCameraImageCapture::CaptureToBuffer);
    imageCapture->setBufferFormat(QVideoFrame::Format_Jpeg);

    camera->setCaptureMode(QCamera::CaptureStillImage);

    camera->searchAndLock();
    imageCapture->capture();
    camera->unlock();
    //testLabel.setPixmap(camView->grab());
}

I think you should take a look at the QCameraImageCapture class.