qxcbdrag lags above high frequency updated drawabled

• create a drawable with high frequency updates (play a video, run glxgears or an xscreensaver hack)
• Find a drag source (listwidget form preview from designer works)
• drag an item

=> Drag is fine until it crosses the high frequent update area. At this point the drag source and X11 will load your CPU and the drag to lag (severely)

The system otherwise remains responsive.

This might be related to the nvidia driver, it is not related to the presence of a compositor.

Attached is a callgrind of dolphin.
Also see https://bugs.kde.org/show_bug.cgi?id=352019