QML thread CPU usage is higher than previous Qt versions

On VxWorks it has been observed that the QML thread is consuming a very large amount of CPU time (between 45-50% per core) compared to previous versions of Qt (5.5.1) and to Qt 5.9 on other platforms. This leads to a big drop in rendering performance (7 fps vs 28 fps with 5.5.1). Suspending this thread seems to work around the issue without any adverse effects and gives ~33-38fps performance like with 5.5.1.

It was noted that some paths changed in Qt, from:

• :/scenegraph/shaders/smoothcolor.vert
• :/scenegraph/shaders/smoothcolor.frag

to:

• :/qt-project.org/scenegraph/shaders/smoothcolor.vert
• :/qt-project.org/scenegraph/shaders/smoothcolor.frag

and this resulted in the same performance between 5.5.1 and 5.9 when tested on a Linux system. However there is a still an issue on VxWorks, giving around ~20-21 fps compared to ~33-38 fps with 5.5.1.

With the thread suspended, frame rate is still down by 1-3%.