By speeding up OpenCart, you offer a better experience for your customers, improve your page speed (and therefore, possibly, your search engine ranking) and make your store appear more stable and professional.
Steps I will send, will help you in :
Leverage browser caching
Asynchronous load addthis script
Leverage browser caching images, CSS, JS file
Globally set domain predix to www to will avoid duplicate content
<<<<<< NOTE >>>>>> This Gig is for basic OpenCart theme only. If you used any third party theme or purchased theme from themeforest, templetmonstor or any other firm, You need to choose Gig Extras