When using QtCreator in tempalte heavy code I noticed that the autocompletion becomes completely unusable.
Every completions for my current project take around 7 seconds which is almost exactly the time how long my project needs to compile.
I think that the results of libclang are not cached correctly.
I am also a user of https://github.com/Valloric/YouCompleteMe which also uses libclang so I expect the performance to be similar.
YouCompleteMe also takes around 7 seconds for the very fist completion, after that the completions seem to be instantly available.