root namespace coding convention in C++

It has nothing to do with parsing speed. C++ uses Argument-dependent name lookup - Koenig Lookup and when you have to make sure that the compiler uses the symbol from the global root namespace you prefix it with ::. If you don't the compiler might also use function definitions from other namespaces when it sees fit (depending on the lookup method).

So, it is better not to do it unless you have to.