This has nothing to do with performance.
However, imagine this: you're utilizing two libraries, Foo and Bar:
using namespace foo;
using namespace bar;
Everything is good, and you can easily use Blah() from Foo and Quux() from Bar.
However, one day you upgrade to Foo 2.0, which now includes a feature named Quux ().
You've created a conflict:
Quux() is imported into your global namespace by both Foo 2.0 and Bar.
Fixing this will take some time, especially if the function parameters are same.
The introduction of foo::Quux() would have been a non-event if you had used foo::Blah() and bar::Quux().