animation runs faster after the qmlpreview refreshed the app

import QtQuick 2.9

Rectangle {
    visible: true
    width: 640
    height: 1000
    color: "blue"


    Rectangle {
        id: testRect
        y: 4
        width: parent.width
        height: 10
    }

    NumberAnimation {
        id: testAni
        target: testRect
        property: "height"
        running: true
        to: 1000
        from: 0
        loops: -1
        duration: 10000
    }
}