How to decreasing the loading time of a website?

use wp super cache plugin

Optimize Caches improve Page Load Time.
Trim Down JavaScript Parsing to achieve Ideal Page Load Time.
Do Away with Redirects and Improve Website Speed.
Minify JavaScript & Style Sheets to Speed Up your Website.

Use a caching plugin if it's a wordpress site.

Methods to decrease page load time:
Optimize images
Minify CSS and JavaScript files
Use content delivery network
Optimize caches.

Remove unwanted javascript file.