Mouse Area: Multiple windows decrease dragging performance noticeably

I'm developing an application consisting of up to six QQuickWindows that may be open at the same time. The rendered scene contains some draggable labels. Each label has its own MouseArea having the label set as drag target.

If there is only one window open, the dragged label does not lag behind the cursor even if the cursor moves very quickly. But when there are multiple windows open, there is a noticeable lag.

To isolate the problem I have prepared a simple scene that can be opened with the qmlscene viewer. The scene consists of a configurable number of windows showing a single draggable rectangle. How many windows will be opened is defined by the num_window property of the root item.

In my test runs the lag got clearly visible with 5-6 windows.