QAccessible causes major performance lost

While experimenting with 5.0.2 under Windows, using Qt 32bit, I can experiment some major performance lost when certain operations with widgets are performed, like opening a large dialog, or when opening or activating large windows.
Compared to Qt 4.8.3, I could measure a speed decrease up to 2x in release mode, and up to 6x times in debug mode.
After debugging, I found that the QAccessible interface is the origin of the problem... I did this:
in qaccessible.cpp
QAccessible::queryAccessibleInterface(...)
I insert "return 0;" at the start...
et voilà, back to normal speed! exactly as in 4.8.3