This is something I'd still like to look at - I think I could optimize the non-scope autocompletion a bit so it doesn't have to govthrough so many symbols (and sort those). The biggest danger is libraries like glib or gtk where all the symbols have the same prefix g_ or gtk_ and are possible candidates for the autocompletion popup.
OK, it's hopefully fixed in the latest commit.
@elextr I still find it a little strange that I get autocompletion results for variables declared after the current line in the function (I would say it's more annoying to see these invalid suggestions than the rare problem when you have variables declared later in the code by mistake). If you use some other editors for C/C++, would you check how they behave in this respect?