Computers are finicky. As stable and reliable as we would like to believe they have become, the average server can cease to function for hundreds of different reasons.
There's no shortage of generic web performance optimization advice out there. You probably have a checklist of things to do to speed up your site. Maybe your hosting company sent you a list of performance best practices.
Drupal has a powerful suite of tools and features for just about any type of website, and eCommerce websites are no exception. Drupal has a very active and healthy eCommerce community that creates some amazing features.