Why is “using namespace std;” considered bad practice?

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, 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 have used foo::Blah() and bar::Quux() then the introduction of foo::Quux() would have been a non-event.