Test it first
Put your address into Google's free PageSpeed Insights tool and look at the mobile result. It shows what is slow and how long visitors really wait.
Too many plugins
Every plugin can add scripts and styles to every page. Shop, slider, form and page builder plugins are the usual culprits. Remove what you do not use.
Heavy images
Photos straight from a camera or phone can be several megabytes. Resize them to the size they are shown and save them in a modern format.
A page builder
Page builders are convenient but add a lot of code to every page. Some sites carry hundreds of files before a visitor sees anything.
Cheap or overloaded hosting
A slow server makes every page slow. If your host is slow before anything is added, no plugin will fix it.
When to rebuild instead
If you have already trimmed plugins and images and it is still slow, the weight is built into the site. Rebuilding it as plain static pages removes the database and most of the code, which is why those sites open almost instantly.
Questions
What is a good load time?
Aim for a page that is usable within about two seconds on a phone. Google's tools score you against targets for loading, responsiveness and stability.
Will a caching plugin fix it?
It can help, but it is a patch on a heavy site, and it is one more plugin to maintain.