@Lodder's answer is probably the best one. However, for those who cannot easily change their template, I recommend trying jQuery Profiles to remove unnecessary scripts and try JotCache as it can work with JCH Optimize to handle the caching better.
Firstly, I'd suggest not using Google PageSpeed, but instead the "Audit" tool build into the Web Developer Tools (uses LightHouse). Simply press "F12" to open the developer tools, then navigate to the "Audit" tab.
The problem with pre-made templates is they tend to use massive frameworks such as Bootstrap, UIKit or Foundation. They'll load the entire ...
Cache clearing instructions can be found in the Joomla! docs here:
Here are some additional resources:
Have you done already these basic checks?
1 - Enable Joomla Caching
This will already significantly boost your page loading times
Go to your plugins and open Page Cache plugin
Usually it shall be already configured like this:
Save and close it
2 - Configure your Cache TTL and behaviour
Go to System - Global Configuration - System(-tab)
Enable the ...