Is there any real difference to the performance when you use Netty and if you don't use it in an application with tens of thousand of connections?
Not really, a good reason to use Netty is to improve the reliability of the connections and leave you to code what the connection does rather than worry about the details of everything which can go wrong. (Often only comes by finding out the hard way)
Netty may help you scale over 1K connections. However if you don't need so many connections you might find that simple code performs best.