Clang Model: code completion speed regression

Qt Creator 3.5.1 on subsequent code completions gets faster with the second code completion.

Qt Creator 3.7.0 (master branch) takes the same amount of time for the same code completion command e.g. "this->".

I used Lyx project to test this regression: git clone git://git.lyx.org/lyx

My CMake configure parameters were:

-DLYX_DEPENDENCIES_DOWNLOAD -DLYX_USE_QT=QT5 -DCMAKE_PREFIX_PATH=c:\Qt\Qt5.5.1\5.5\msvc2013\lib\cmake\

Went to the last method in Text3.cpp – Text::inDescriptionItem and added a this->. Took like ~6 seconds to
get the popup (older i7 2.6GHz CPU). During the ~6 seconds I didn't see any Windows wait cursor.

On Qt Creator master on every this-> I get like ~6 seconds.

With Qt Creator 3.5.1 the first time is slow the next times are way faster, like one second.

I've used the same libclang.dll for both Qt Creator 3.5.1 and Qt Creator 3.7.0 (master), the latter shouldn't be ~6x slower.