This is not related to performance at all. But consider this: you are using two libraries called Foo and Bar:
using namespace foo; using namespace bar;
Everything works fine, and you can call
Blah() from Foo and
Quux() from Bar without problems. But one day you upgrade to a new version of Foo 2.0, which now offers a function called
Quux(). Now you've got a conflict: Both Foo 2.0 and Bar import
Quux() into your global namespace. This is going to take some effort to fix, especially if the function parameters happen to match.
If you had used
bar::Quux(), then the introduction of
foo::Quux() would have been a non-event.