This is presumably the way to take the most time to work on, but the profit can be very high. Speed of website pages can be slowed down by different things, some obvious, some not. One of the most valuable methods to analyze your site is Google tools like a lighthouse or PageSpeed Insights. They are separate tools, but PageSpeed uses parts of Insights Lighthouse, which comes down to precisely what you prefer. Get to know one of them, and take advantage of their reports to improve your site's performance. Like a mobile-friendly site, page speed has become an essential factor in your domain authority.
If your site has not yet been optimized for mobile usage, not only will it hurt your search rankings (since Google supports mobile-friendly sites), but your visitors to your site Users will also lose. So, go to Google's Mobile-Friendly Test, and then run a test for your domain. After that, Google will give you a detailed report on how mobile-friendly your site is and what you can do to make it better.