Speed Optimization of the Web Application

I want to speed optimize one of the web application.The application has not much images,CSS and JS.Not much external JS as well.But the data which is showing in the application that data is coming from Salesforce API. And when i try to access the page in which data is coming from Salesforce API then the page takes too much time to load.I have tried to check the speed of the website using "pagespeed insight" then it throws the error which i have attached below in the image.

Note : May be this question is silly to all but i am not getting how to do speed optimization of the web application.

Thanks in advance!