Better performance when importing just a part of a library?

They're entirely different. A using directive of that form only tells the compiler to make members of the given namespace available as simple names. It has no effect at execution time - if you didn't have any using directives and just referred to everything by the full name, you'd end up with the same code. As "nested" namespaces aren't imported, just using SportsStore won't import members of the SportsStore.Domain.Entities namespace.

(Using directives also make extension methods declared in types within the given namespace available, btw.)

As a rule of thumb, have using directives for everything you need to (in order to keep the bulk of your code simple) but no more than that, just to avoid cluttering your code unnecessarily. Visual Studio has a command to sort using directives alphabetically and remove unnecessary ones - I have that bound to Ctrl+Shift+U which I use fairly frequently.