Improve page loading times of our pages

we are a start up building a large global platform for local social ecommerce. Our platform is built on LAMP architecture using yii2. We also have large number of strandardised API ( central functions) calls based on which each of our page loads. through this design, while our functionality is working but page loading time is high. we want each page to load in max 4-6 seconds. Some of our end users also have slow network speeds so we want to optimise the loading of our pages for those speeds also. This is like a test job for the new developer and we will initially like to fix 3 pages with the help of this freelancer. Once this is achieved, we will extend this engagement to fix all other pages.