Retested the app (QtQuickTimerTest) on a different machine. Now with again different results:
Both NVIDIA and Intel are fine when using OpenGL with the threaded render loop (as is the new default in Qt 5.5). Cannot reproduce the previous 1300-1400 results.
When switching to ANGLE which in turn triggers the windows render loop - or just forcing the windows render loop with desktop GL - it all goes wrong (300 ms instead of 1000, etc.) as shown above.