My FPS fall drastically when recording a video - opencv

Question

I'm using OpenCV to record video for a Full HD Camera Running at 30 FPS. I know the FPS of the camera because I measure it by counting the number of valid frames (this also coincides with the specifications). I do this by using a 5 ms second timer. Here is the code that I run each 5 ms.
cv::Mat frame;
if (capture.read(frame)){
showCurrentFrame(frame);
fpsCounter.newFrame();
ui->labelVideo->setText("Video (" + QString::number(frame.cols) + "x" + QString::number(frame.rows) + ")");
ui->labFPS->setText("FPS: " + QString::number(fpsCounter.getFPS()));
if (isRecording){
recorder << frame;
fpsRecordCounter++;
}
}
I set the FPS of the recorded video to 30 when I press the "Start Recording Button"
recorder.open(currentVideoFile.toStdString(),
VIDEO_CODEC_FOURCC, // Tells it to record MJPEG
VIDEO_REC_FPS, // Define with 30
frameSize)){ // 1920x1080
I've developed my Program in my workstation which runs Centos 7 using Qt and OpenCV 2.4.5. When I record in the desktop PC the FPS show consistently to be about 30.
However, this needs to record from a moving car. So I copy-pasted the code AS IS into my laptop and compiled it with zero issues.
My laptop uses Debian Testing and OpenCV 2.4.9. It is HERE that the slowdown is observed.
Since I'm using Qt I need to process the cv::Mat in order to display it. I do this in the showCurrentFrame function.
If I deactivate this function when recording I get maybe 23 FPS (and can't see what I'm recording)
If I leave it as is in the code above I get about 16~17 FPS.
The first thing that I thought was that my computer was not powerful enough, but it shouldn't be. This is the model:
https://www.asus.com/Notebooks/ASUS_VivoBook_S550CA/specifications/
It's the I5 variant with 500 Gb of HDD.
So I'm at a loss. Is it some sort of bug on OpenCV that was introduced in newer openCV versions or is it simply that my laptop is not powerful enough?