CPU load strongly increasing when constantly calling update() SLOT of few separate QQuickWindows

I found that update() slot consumes more and more CPU power it time when running in several windows.

I wrote a simple program so everyone could check what am i talking about:

    import QtQuick 2.3
    import QtQuick.Controls 1.2
    import QtQuick.Window 2.2
     
    ApplicationWindow {visible: true; width: 640; height: 480;
        Window{  height: 100;  width: 100; visible: true; objectName: "w1"; id:w1; }
        Window{  height: 100;  width: 100; visible: true; objectName: "w2"; id:w2; }
        Window{  height: 100;  width: 100; visible: true; objectName: "w3"; id:w3; }
     
        Timer {
            interval: 30
            repeat: true
            running: true
     
            onTriggered: {
                w1.update();
                w2.update();
                w3.update();
            } 
        }
    }

for my system this code increases cpu load lineary from 10% to 60% in 100 second. screenshot is attached.

It happens only if updating is in few separate windows.
If i completely reload animated windows with Loader{}, everything resets and CPU load starts to increase from beginning.

I had already tried to periodically call releaseResources() method and resetOpenGLState() method of QQuickWindow.

ALso very interesting is that when I apply putenv("LIBGL_ALWAYS_SOFTWARE=1"); in main.cpp this issue goes away and i have CONSTANT low cpu usage, but hardware acceleration is turning off.

Also when QT_OPENGL_NO_SANITY_CHECK=1 CPU load stay constantly low, until application crashes.