Hi there,
I’ve checked the site you’ve linked and it doesn’t seem to be using any optimization plugin.
For starters, you’ll have to install one and start w/ the most simple settings first.
You then tweak your optimization plugin’s settings up until you get the best result out of it.
As for plugins specifically, most of our users usually use WP Rocket, Litespeed cache or Autoptimize.
If you plan on using Autoptimize, check this – https://docs.generatepress.com/article/configuring-autoptimize/
As for the other plugins, you can contact their support or try reading through their documentation to learn about their settings. 😀