Customers don't like to wait, so why keep them waiting with a slow store?>
I will share with you the top three reasons that I have found are behind most performance issues with Bigcommerce stores. Number two and three are more technical so I recommend looking for a developer to help you make the necessary changes.
1. Using very large images for the carousel/slider plugins
This usually manifests itself as a slow loading home page that gets a bit faster the second time around thanks to your browser's cache. Make an effort to keep the number of slides down to a minimum. A slider should have no more than four or five image slides. You should try and keep the image size to around 500 kilobytes or less. Above that and you will definitely notice a slow down in the loading times.
2. Duplicate panels and snippets
This problem is harder to spot because the duplicate panel is usually hidden from the page and only appears in the source code. You need to look in the template files for panels that have been duplicated on the same page.
Here's what to look for:
%%Panel.CartContent%% <!-- %%Panel.CartContent%% -->
The HTML comments around the second panel do not prevent the Bigcommerce template engine from processing the panel. This means you end up with two cart contents on the same page: one that shows on the page and another that is hidden and viewable only in the source code. The same can happen with snippets.
3. Using load() to grab data from multiple pages
jQuery's load function fetches data from the server and returns it to an HTML element. Think of it as a way to grab data from different pages of your site without actually browsing there yourself. The problem with load comes when it is used too often and usually not for a good reason. An example of this is a script that uses load for every single product on a category page to extract the product description. This is not needed because the product description can be made available on the category page by turning on the list view option from the control panel. Think about it this way: if a page takes roughly half a second to load, and you are loading ten pages in the background then you have increased your store's loading time by five additional seconds. Very few customers are going to wait around that long.
I hope the list was useful to you in improving your store's loading time. The faster your store loads, the more sales you make. Happy selling!