Complex structure of multiple aliasses and slow browsing

This webshop is located on a Plesk profile "[login to view URL]" It has 2 aliasses: [login to view URL] [login to view URL] When visiting [login to view URL], it forwards to bartokshop.be. When clicking on anything but the homepage, you're going to a [login to view URL] page/product. Due to copyright issues, the shop needs to use bartokshop.be. Nothing needs to be moved, only configuration files need to be altered. The other problem is, the site is very slow. It's a modified oscommerce shop and seems to be very slow on every page. Google Pagespeed says "Improve server response time" but the server is responsive on every other domain.