Performance issue without multisampling with opengl

Firstly, I didn't set the samples of QSurfaceFormat to QQuickView.
Reference to the doc, the multisampling is default disable. http://qt-project.org/doc/qt-5.0/qsurfaceformat.html#samples

My sample is easy, just drag the rectangle by MouseArea.
I found there are two issue will cause the performance higher :
1. If the panel is wider (ex: 1400x900), the CPU rate is high than small area (360x360).
2. If mouse control be contained in the Rectangle component, the CPU is high than the component is Item.

Then I set the sample to 0 (Disable the multisampling). The CPU rate is lower.
My question is, if default is disable, why the performance has apparently different after setting ?

Thanks