In real world, I trade fast autocompletition for ignoring namespaces.
BTW, it is still the same issue: You can do this "correct way": that means running preprocesor on the file each time you request autocomplete and each time you leave the file (to have the global map of symbols).
Preprocessor itself on average file runs for 2-3 seconds. It creates an output that has about one million of lines (because of includes) - then you have to parse this file.
Plus, autocomplete would cease to work in certain header files too...
That is why I believe it is essential to cheat here.