If you host your internet sites on a dedicated server, you would expect that they'll perform ultra fast and that the access speed to them shall depend entirely on the Internet connection of the site visitors. Nevertheless, this will not be the case in case the hosting machine has lousy network connectivity or uses a network card, which just can't deal with high volume of site traffic. If this is the case, it shall take a very long time for your sites to load if many people open them at the same time or visitors could see error messages. Therefore you could lose visitors as most probably many people will not revisit your website if they have experienced difficulties or slow loading speeds. That's why you need to pay attention to the network components of any new server you purchase and not only to the main hardware like Processor, Random access memory or hard disk.
