Understanding the different warning messages from PageSpeed Insights speed report

The first thing you need to know when running Google PageSpeed Insights (a.k.a. Google Lighthouse) is that it only provides general information. This report cannot identify if a certain code is really necessary or not, and this is especially important if you use third-party apps, as it might consider that the app code is not essential and claim that it could slow down your store.

Having this in mind, we will explain below why our app code may appear in some of the warning messages you can see in Google PageSpeed Insights:

Warning "Remove unused JavaScript"

The report detects our code as "unused code." We can definitely say this is a false positive. Bear in mind our app code must be present in order to display on the cart page the different shipping rates you have configured in your Shopify admin.

Just for you to know, we are using a Shopify programming feature called ScriptTag, which allows our app to be injected dynamically by Shopify when a page loads, to prevent theme template modification. Apart from this, the code contained in the loaded script is reduced to the bare minimum in order to minimize the impact of our app loading.

Warning "Serve static assets with an efficient cache policy."

Our resources are cached for about 1 minute (app data and preferences) and 1 hour (app script).   

The former is necessary to ensure that the app widget shows the most recent data, allows you to perform changes in your app preferences, and have all changes reflected quickly.   

The latter is convenient to always have the latest version of our app, ensuring compatibility with the changes in the Shopify platform.

Having a different cache policy would cause more harm than good; for instance, our app would not show the most recent changes on the shipping rates configured in your Shopify admin. For that reason, a shorter cache period guarantees that your correct shipping rates are displayed at all times.

Warning "Reduce JavaScript execution time"

In order to display our app on your store, we need to insert the app JavaScript code. The report detects that our app code is loaded and may recommend removing it because the report does not know what it does. However, our app code is essential to show the app.

Please note that our JavaScript files are compressed, minified, and served from a top world CDN to offer the best possible user experience to your customers.

Warning "Avoid long main-thread tasks"

In order for our app to be displayed and continue working while your customers are browsing your store, it needs to remain active, and this is what the report identifies as a "long task". However, most of the time our app is doing nothing and consuming no resources but it must remain loaded to interact with your customers' actions.

Warning "Avoid chaining critical requests"

The reports warns that it has detected several scripts running before all page resources have finished loading. The report does not know if some resources are more important than others, so it displays a generic warning.