If you have a store with large no of products and categories then I am sure that you are facing an issue with website speed.
You have to inactive unnecessary entry in database like product / categories count in header. Use indexing in your database.
If you are using lots of images for every product then use different domain for images, you can create one sub domain example img.yourlink.com then u got edit config.php at public html and also config.php at admin folder.
Define(‘HTTP_IMAGE’, ‘http://image.yoursite.com/’); or define(‘HTTPS_IMAGE’, ‘http://image.yoursite.com/’);
enable Content compression mode.
Add Expires headers
Compress all external link
There is lots of other technique use by professional OpenCart Development Company. You can also take a support from this company
You can use some existing extension available in market or develop your own extension by Web Development Company.