What exactly is wrong with using namespace std (if you're only using std)?

What exactly is wrong with using namespace std (if you're only using std)?

I've been told not to recommend using namespace std; to beginners, but I
never found any evidence why it's bad. It seems to start causing problems
only if using more than one namespace.
Can someone tell me what possible problems can happen if only including
one namespace, in this case std?
This is not a subjective question, I need an exact answer why it's bad, in
terms of performance and being understood by the compiler.
This is not a duplicate, the other question deals with all types of
problems that can occur. I'm asking only about the case where you need to
use one single namespace, in this case, std.