ParticleSystem greatlty consumes CPU when window is covered by other window

I used ParticleSystem in my QtQuick2 program. Normally it has 2% CPU usage. It's okay. But when I move another window to COMPLETELY cover my QtQuick2 window (which used ParticleSystem), the process uses 25% CPU at once. I use a quad-core CPU, so 25% CPU looks like the program running in a dead loop.

As long as the QtQuick2 window is not completely covered by the other window, the CPU usage will not rise.

Is it a bug of Qt?

It's easy to reproduce, all my program using ParticleSystem have the same problem, as well as the EXAMPLES in Qt folders.

Below is my code:

import QtQuick 2.0
import QtQuick.Particles 2.0 
 
Rectangle {
 width: 1000
 height: 800
 color: "grey"

 ParticleSystem{
  id: particles
 }
 ImageParticle{
  anchors.fill: parent
  system: particles
  source: "pics/particle.png"
  colorVariation: 0.6
 }

 Emitter{
  anchors.fill: parent
  system: particles
  emitRate: 5
  lifeSpan: 2000
  size: 5
  sizeVariation: 2

  acceleration: AngleDirection { angle: 90; angleVariation: 360; magnitude: 20; }
  velocity: AngleDirection { angle: -90; angleVariation: 360; magnitude: 10; }
 }
}

I'm on Windows XP, use the default Qt5.0.1 binary in "DOWNLOADS" page (so I use ANGLE).

I've also tested it on Windows 7, it works well. Maybe only Windows XP have the problem?