Its been a long time since this thread was started but I think it makes sense to post the updates here. I put together a basic project with native OpenGL and also using Qt's QOffscreenSurface and QWindow classes.
The github repository can be found here:
In this setup there is a single renderer that render to an FBO texture using its own thread and 3 display windows that share the GUI thread with all of them sharing OpenGL contexts together.
On my machine I see a clear lag with VSync on when there are 2 or more display windows. Turning VSync off clears the lag. The calcualted FPS values do not indicate any issues in either case which doesn't help but I see a clear difference when I start the application with say 2 displays and then close one of them.
I have my internal laptop display and an RCA RT1970 (both running at 1920x1080 - 60Hz) and both connected to my integrated Intel HD 530 GPU. My discrete GPU is an NVIDIA GeForce GTX 950M and I'm running Qt 5.12.0 MSVC2017 64 bit.
I'll dig in further myself also with other machines but if anyone has any ideas on what could be causing this, it would really help debug further.