DropShadow with static source is faster when using “cached:true”

[Stefan Monov76]

I'll begin with my testcase. It creates 21 unchanging shadowed blue rectangles. It also creates a 1x1px Canvas3D repainted constantly, so I can check how often it manages to get repainted with all the other stuff going on (Canvas3D has a built-in fps property). When cached: true is set on the DropShadow items, I get 60 FPS. When not, I get 30 FPS. But what I expect is to get the same FPS in both cases, since I don't expect the shadows' blur to ever get recalculated, considering that the source rects never get updated.

main.cpp: (trivial)

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    
    return app.exec();
}

main.qml:

import QtQuick 2.5
import QtQuick.Window 2.2
import QtCanvas3D 1.1

Window {
    visible: true
    width: 800
    height: 600
    id: window
    
    Column {
        Text {
            text: canvas3d.fps + " FPS"
            font.pointSize: 18
        }
        Flow {
            width: window.width
            spacing: 10
            Repeater {
                model: 21
                
                ShadowedItem {
                }
            }
        }
        Canvas3D {
            id: canvas3d
            width: 1; height: 1 // nonzero size so it can be redrawn
            property var gl;
            
            onInitializeGL: {
                // should get and save context, otherwise FPS isn't measured for some reason
                gl = canvas3d.getContext("canvas3d", {depth:true, antialias:true, alpha:true});
            }
        }
    }
}

ShadowedItem.qml:

import QtQuick 2.0
import QtGraphicalEffects 1.0

Item {
    width: 100
    height: 100
    
    Rectangle {
        anchors.fill: parent
        id: rect
        visible: false
        color: "blue"
    }
    
    DropShadow {
        source: rect
        anchors.fill: rect
        cached: true // !
        radius: 8
    }
}

Any idea on the difference in performance?