X11 vs XP graphics performance on QT 5.2.1

tom.abcd

I have a simple app that has three QLabels layered with 1 on the bottom, 2 on top of 1 and 3 on top of 2 and 1.
In the first Qlabel I load a JPG image. In the second I load a PNG image that has an alpha channel.
In the third Qlabel I am showing an incrementing number. I used a timerEvent the runs every 30 msec that increments a number converts it to a string and put the string in to the label

void ImageTextTest::timerEvent(QTimerEvent *event)
{
counter++;
ui->imageLabel_3->setText(QString::number(counter));
}

I have built my app in Windows and on Linux and then run it on my test unit.
On the test unit I can boot either XP or Linux (lubuntu).

If I run my app on XP and look in taskmanager it appears to take almost no cpu load.
If I run my app on Linux and use Top it seems to take about 21 to 23 % on one core for the app plus another 7 to 8% for Xorg.

I have tried using -graphicssystem (openvg, opengl, raster) on the Linux box but did not see any performance difference.

So my questions.

Is lower QT graphic performance typical on Linux vs XP?
How do I tell which graphicssystem the QT app is using?
Is there some test I can run to see which graphics systems are supported on a PC?

Thanks
Tom