webpagetest.org is a website that can check your website for performance. It will provide you an optimization checklist which suggests you what all changes you need to make for better performance.
Also rate your website in different sections
You can see two analysis one is for First view of the page and the second one is the repeated view of the same page. This website will tell you how you can fix the things for better performance like
Use one CDN for all static assets
Combine static CSS and JS files