How to Fix initial loading time TTFB in codeigniter?

My company website www.carlo.in is somehow taking more than 15 seconds for TTFB. I have checked everything, GTmatrics showing 82% PageSpeed Score but still Fully Loaded Time is 19.9s. I have checked the server side and its working smoothly because there is one other domain which is in the same server working fine. Actually, www.carlo.in is built on PHP framework Codeigniter and I think that is happening because of the initial loading of main-controller which is slow. And another website in the same server build on Wordpress working fine , i don't know what to do now please someone help, I might lose my job due to this.