If your website is suddenly loading slowly, this could be caused by a high load on the server. This happens when the server doesn't have enough power to keep up with the amount of requests it is receiving. This results in the server load going up, and pages to open slower.
If it has been more of a constant issue there are several things you should evaluate. First check the size of your index page. Does it have many images? Does it request lots of information from the database (multiple product listings)? Does it request information from other websites (verification images, RSS feeds, external services)?
Your MySQL may also be having some slow queries if your databases are very large. This can happen specifically with X-cart and the stats adding up over time. You can see instructions on how to clear these stats here. It is also recommended to optimize your databases through phpMyAdmin.
These all can create a slow down on your website. A good check to do is to login to cPanel and browse around, and see if this is slow as well or if it is just your website. Please open a ticket if, even after optimization, you are having unnacceptable loading times.