Magento – Magento 2 – Best practice for extending customer entity

I'm looking for the best practice for extending the customer entity.
I want to add some custom fields (user_name, company_infos, etc.) and I don't know if it's better to extend the customer entity with the new attributes or creating a new table to store the additional data. The goal is to visualize the custom data on the product detail page.
So what do you think, which is better the solution to get the best performance?

Thanks.

Marco

Update:
I followed the hints below, but the extension Attribute won't be saved.
Do you have any suggestions to solve it?

I created an extension_attributes.xml file:

<config>
    <extension_attributes for="Magento\Customer\Api\Data\CustomerInterface">
        <attribute code="user_name" type="string">
           <join reference_table="customer_extension" reference_field="customer_id" join_on_field="entity_id">
              <field>user_name</field>
           </join>            
        </attribute>
    </extension_attributes>
</config>

And an Observer which is registered on the "customer_register_success" event:

<?php
namespace My\Module\Observer;

use Magento\Framework\Event\ObserverInterface;

class AfterCustomerSaveObserver implements ObserverInterface {

    protected $_logger;
    protected $_customerExtensionFactory; 
    protected $_customerRepository;

    public function __construct(
            \Psr\Log\LoggerInterface $logger, //log injection
            \Magento\Customer\Api\CustomerRepositoryInterface $customerRepository,
            \Magento\Customer\Api\Data\CustomerExtensionFactory $customerExtensionFactory
    ) {

        $this->_logger = $logger;

        $this->_logger->addDebug('AfterCustomerSaveObserver_Constructor_Begin');

        $this->_customerRepository = $customerRepository;
        $this->_customerExtensionFactory = $customerExtensionFactory;


        $this->_logger->addDebug('AfterCustomerSaveObserver_Constructor_End');
    }

    public function execute(\Magento\Framework\Event\Observer $observer) {

        $this->_logger->addDebug('AfterCustomerSaveObserver_Execute_Begin');

/*      try { */

            $event = $observer->getEvent();
            $customer = $event->getCustomer();  

            $customerExtension = $customer->getExtensionAttributes(); 
            if ($customerExtension === null) { 
                $customerExtension = $this->_customerExtensionFactory->create(); 
            }       
            $customerExtension->setUserName("Test123"); 
            $customer->setExtensionAttributes($customerExtension); 

            $this->_customerRepository->save($customer);
            //$customer->save();

/*      } catch (\Exception $e) {
            $this->_logger->critical($e);
        } */

        $this->_logger->addDebug('AfterCustomerSaveObserver_Execute_Begin');
    }

}