Start any Qt3D application and leave it running. Lock computer and wait few seconds. Log back in and observe CPU load from task manager performance tab. On my machine it goes to 25% i.e. 2 threads 100%.

Now the reason. We run rendering loop which waits for vsync. My theory is that in lock screen vsync gets disabled and our rendering loop starts to run as fast as it can. I noticed this with my laptop as the fans start running full speed after few minutes into lockscreen.

You can debug the behavior more easily with disabling vsync manually or if you disable aero (also disables vsync).

Should we have a backup timer for the loop, should we stop rendering while in lockscreen or leave it as it is?