Website Waiting Time Is Very Hogh

Hii Frnds,
My Domain Is https://www.scienceofwonder.org
When I Check on tools.pingdom.com I See That Waiting Time Is Around 4 Seconds. Which Is Very High. I Am Hosting Website On My VPS Server. Its Only Around 10 Days , My Website Is Running.
When I Contact With Server Team , They Say Me That Its Not A Server issue. You Have To Optimize Your Script. You Have To Contact To WordPress For Your Resolution.
Please Help Me , How I Reduce My Server Response Time.
I Have No Knowledge Of Coding, Is This Is The Problem Here.
But As I Hear That WordPress Require Little Knowledge of Coding.
Please Please Give Me Solution As Soon As Possible.

Thanks In Advance.