:-)

It is not April Fool's day!

Installing Qt5 libraries will not make the device faster. Those libraries are not used unless there are newly ported and compiled applications that make use of those libraries. None of the software on N9 uses qt5 libraries, they would need to be recompiled to use qt5 libraries.

Even if you tried to replace the existing qt4 libraries with qt5 versions, it would not work. Qt5 is not binary compatible with qt4 so the applications would crash unless recompiled.