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.

    To increase Massive speed you have to some database level changes.

  • 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.