[Reg Qt5.7.1-Qt5.8] QMenu became multiple time slower than early version.

QMenu is very slow with with Qt5.8 and 5.9.

Attached example reproduces this problem.
Run the attached example and this will show the time it took add menu in window Title.
Running with Qt5.7 or earlier version it takes around 0.25sec but with Qt5.8 or higher it takes around 25 sec.