Sometimes CPU usage is worse than Qt5.1.1 on Qt5.2.0 beta1 on BeagleBoard-xM

I'm using self-built Qt5.2.0 beta1 configure and built using same environment on BBxM,
and found sometimes cpu usage (by 'top' command) is around 20% worse than Qt5.1.1.

Attached qml file is which I tested, and FpsItem came from cinematic experience of Boot to Qt.

Here are some results when screen size is 800 x 480.
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FpsItem is NOT visible:
Qt5.1.1: around 4%
Qt5.2.0 Beta1: around 3%

FpsItem is visible:
Qt5.1.1: around 30%
Qt5.2.0 Beta1: around 53% <-- this is what I report

Radius of Rectangle is 0:
FpsItem is NOT visible:
Qt5.1.1: around 4%
Qt5.2.0 Beta1: around 3%

FpsItem is visible:
Qt5.1.1: around 36%
Qt5.2.0 Beta1: around 28%
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