Google PageSpeed Insights Tool
Google PageSpeed Insights is a website performance monitoring tool. We recommend that you use it as default because it shows how Google rates your store. Since the last update, it has become more informative and powerful due to Google Lighthouse data used. At the same time, it has become more demanding for mobile versions.
By the way, you can use the Google Chrome Lighthouse tool locally in the DevTools panel of the Audit tab.
GTmetrix Speed Test
GTmetrix also provides you with a lot of reports and detailed explanations of your store performance. You can run different tests and get useful recommendations concerning waterfall reports, timings, etc.
Shopify Speed Optimization Approach
Let's see how we can receive more by doing less. When you run Google PageSpeed Insights Tool and GTmetrix, you get a lot of recommendations. However, don't try and heed them all. For example, they offer to eliminate render-blocking resources. It means you have to move all CSS and JS to the bottom of the page. But if you do that, the page layout will break while loading, and JS may crash as well. You will probably spend much time trying to fix it, so, obviously, it is not the best move. But in this case, what should you do first?
Reduce Image Size
The size of each image should be the same as you want to load. There are two kinds of pictures on Shopify. The first one is photos; I wrote a guide for their optimization. You can read it here. The second one is icons and logos. You should ensure that SVG or font icons are used for it when possible. Vector images, easily scalable and lightweight, look great on any display.
Image optimization is the primary way to reduce page size and increase speed ratings. So now we have the first thing to do.
Uninstall Unused Shopify Apps
Most Shopify apps load some scripts to your store. This process is usually asynchronous and does not influence the visual speed of page loading. However, each script may load more scripts, analytics, and new assets, which can decrease page rates or even freeze the page.
Review your app list and uninstall everything you haven't used for a long time. It's better to remove all the applications you can get by without even if you use them from time to time.
Cloudflare is a tool for protecting and accelerating websites. It caches assets, minimizes CSS and JS, prevents DDoS attacks, and does many other things that automatically speed up your store. You can read here how to use it with Shopify. The SSL certificate should be issued within 24 hours. If your store is already live, Cloudflare will provide you natural performance growth.
There is no silver bullet that will solve all the performance issues at once. Depending on your requirements, you can spend weeks working on it. But, it is possible to achieve reasonable rates and page speed without spending much time and resources if you know how to prioritize your work. Focusing on the most important things first is the best way to meet your business needs.