Integrated code completion fails or works partially

Qt Creator's integrated C++ code model / code completion doesn't work or works partially when a project becomes larger.
I am providing test project where the code completion for QOpenGLFunctions_3_3_Core doesn't work at all using the Qt Creator's integrated C++ code completion:
https://drive.google.com/file/d/0B2SoBuawTfgOSHYxRERvTmVsVTQ/view?usp=sharing

I've written a forum topic about this:
http://qt-project.org/forums/viewthread/43068/
... and there have been discussions in the Qt Creator's mailing list as well:
http://lists.qt-project.org/pipermail/qt-creator/2014-May/003641.html

There is a bugreport that target's Qt Creator's code completion problems partially:
https://bugreports.qt.io/browse/QTCREATORBUG-8937

A possible cure for the problem is to enable the Clang's C++ code model and use it. It always completes code but it is awfully slow. Explicitly requesting the code completion with Ctrl + Space or implicitly triggering it by typing, results in several seconds waiting time - 2 to 4 seconds for the code completion to arrive.
I am using the following machine:
CPU: Core i7 2600 3.4 GHz
RAM: 16 GB
SSD: Samsung SSD 840 Pro

So, hardware shouldn't be the reason for the slow Clang's code completion.

The best thing about Qt is developer experience - the joy, convenience and motivation to code. Bad code completion ruins all of these.