I don't know if it's a bug but if I use multiple GraphicsViews with one QGraphicsVideoItem each and play a video with QMediaPlayer the GUI gets really slow.
Using QVideoWidget I can easily play 20 videos without blocking the GUI, but with windows backend the position label updates every 1 second. With the default backend there is no problem when using QVideoWidget.

With QT_MEDIA_BACKEND = windows and using 5 GraphicsViews the position label updates about every 900ms.
With 4 GraphicsViews every 700 ms
With 3 every 550 ms
With 2 every 400 ms
With 1 every 200-300 ms

With the default backend the position label updates much faster,
but on both backends with 5 views the GUI is completely unresponsive, I cannot even close the application, but the video plays.
With 3 views I can use the GUI ,but it's sometimes more responsive and sometimes less.
With 2 views it seems totally fine, but the windows backend is a little bit less responsive
Updating the QGraphicsScene manually for each frame did not help.