Optimizing Performance in Next.js: Strategies for Faster Apps.

Looking to supercharge your Next.js application's performance? Dive into these expert strategies for optimizing speed and efficiency to create lightning-fast apps.

Introduction

The performance of a web application often takes a backseat in development until its sluggishness becomes too noticeable to ignore. In my experience working on numerous apps over the years, I've found that checking the app's performance during development isn't always a top priority. It's a common oversight among frontend developers to prioritize building components and refining the platform's appearance, relegating performance considerations to later stages.

However, this approach can lead to challenges down the line. While it may temporarily address immediate development needs, neglecting performance optimization can result in a bloated codebase and sluggish user experience as the app scales. As the codebase grows, improving performance becomes increasingly difficult and time-consuming, underscoring the importance of integrating performance considerations throughout the development process.

Optimizing Process

To embark on optimizing your Next.js application, particularly one built using the pages router, it's vital to comprehend the size of both the First Load JS for individual pages and the cumulative First Load JS shared by all pages. Often, developers may overlook the significance of the JavaScript volume loaded on each page and the combined JavaScript shared across all pages during development. You can easily assess these metrics by building your Next.js app, such as through the pnpm run build script. For a detailed examination of your JavaScript bundles, Next.js recommends utilizing the @nextjs/bundle-analyzer tool. This invaluable resource offers insights to refine your application's performance and streamline its JavaScript delivery, ensuring a faster and more responsive user experience.

First Load JS : First Load JavaScript refers to the JavaScript code executed when a web page is accessed for the first time by a user. In the context of Next.js or similar frameworks, it specifically denotes the JavaScript file associated with a particular page route that is fetched and executed upon initial navigation to that route. This JavaScript typically handles tasks such as initializing the page's content, fetching initial data, and setting up event listeners.

First Load JS Shared by All : First Load JavaScript Shared by All refers to JavaScript code that is common to and executed on the first load of every page within a web application. In Next.js, for instance, this shared JavaScript logic is often placed in the _app.ts file. It encompasses tasks that need to be performed universally across all pages upon their initial loading, such as initializing global state, configuring analytics, or setting up common UI components. This shared JavaScript ensures consistency and efficiency in the application's initialization process across different routes and pages.

This is an example of the JavaScript that loads first for individual pages and the JavaScript that loads first and is shared by all pages.

Reducing First Load JS Shared by All

In the quest for high-performance web applications, minimizing the JavaScript (JS) executed on the initial page load becomes paramount. Termed as "First Load JS Shared by All," it encompasses the JS code executed universally across every page load, directly impacting the application's speed and responsiveness. To achieve optimal performance, meticulous attention to reducing this shared JS payload is essential.

Identifying Unnecessary Packages: The journey to streamline begins with a thorough audit of your application's dependencies. Eliminating unused packages not only reduces the overall bundle size but also trims down the first load JS, ensuring only essential code is delivered to the client's browser.

Optimizing the _app.js Component: At the heart of Next.js lies the _app.js (or _app.tsx) component, orchestrating the initialization of every page. Here, each import contributes to the collective first load JS. Scrutinize each import meticulously, ensuring it's indispensable for all pages. Context providers, global CSS, and other shared resources find their home here. However, imports exclusive to individual pages should be relocated to their respective components, sparing the global bundle from unnecessary weight.

Contextualizing Import Usage: The strategic allocation of imports plays a pivotal role in minimizing first load JS. Imports catering to specific pages or components should be localized, invoked only when required. This granular approach optimizes resource utilization, preventing redundant JS execution across unrelated page loads.

By meticulously curating dependencies and refining import practices within the _app.js component, the first load JS shared by all can be significantly diminished. This disciplined optimization not only accelerates initial page loads but also cultivates a streamlined, responsive user experience throughout the application's lifecycle.

I managed to decrease the shared first load JS from 249kb to 122kb, which is a significant improvement.

Reducing the Initial Load of JavaScript on Pages with Heavy Initial JavaScript Loading

In scenarios where certain pages within your application necessitate a substantial amount of initial JavaScript loading, optimizing their performance becomes imperative. These pages, often laden with intricate functionality or rich media content, can suffer from prolonged loading times, hindering user experience. To mitigate this challenge and enhance the responsiveness of such pages, a systematic approach to reducing the initial load of JavaScript is indispensable. Below are strategies I employed to alleviate the burden of heavy initial JavaScript loading for an application:

Analyzing JavaScript Bundles with Next.js Bundler: Next.js provides built-in tools to analyze JavaScript bundles, enabling developers to gain insights into the sizes of their bundles across the entire application. By leveraging Next.js's bundler, you can assess the size of individual JavaScript bundles and identify opportunities for optimization. This granular analysis empowers developers to pinpoint areas of concern and optimize their application's performance by minimizing the size of JavaScript bundles, thereby improving load times and enhancing user experience.

Code Splitting and Dynamic Imports: Implementing code splitting techniques and leveraging dynamic imports enable the selective loading of JavaScript modules based on user interactions or specific page requirements. By breaking down the JavaScript payload into smaller, more manageable chunks, the initial load is expedited, allowing critical page content to render swiftly while deferring the loading of non-essential scripts until they are required.

Lazy Loading Assets and Components: Adopting lazy loading for assets such as images, videos, and components allows for their deferred loading, reducing the initial JavaScript payload. With lazy loading, resources are fetched asynchronously as the user scrolls or interacts with the page, optimizing bandwidth usage and expediting the initial rendering process.

Optimizing Third-Party Libraries and Dependencies: Third-party libraries and dependencies can significantly contribute to the initial JavaScript load. Auditing and optimizing the usage of external libraries, opting for lighter alternatives where feasible, and selectively loading dependencies based on page requirements can help trim down the initial JavaScript payload, fostering faster page loads and improved performance.

Consider leveraging platforms like Bundlephobia to analyze the size of your library dependencies. This allows you to make informed decisions about which libraries to include, opting for lightweight alternatives to minimize the initial JavaScript payload and improve overall application performance.

By meticulously applying these strategies, the initial load of JavaScript on pages burdened with heavy initial JavaScript loading can be substantially reduced. This concerted effort not only enhances the user experience by expediting page rendering but also fortifies the application's overall performance and responsiveness.

Conclusion

In conclusion, optimizing performance in Next.js is paramount for delivering fast and seamless web experiences. By implementing strategies such as reducing first load JS shared by all, optimizing pages with heavy initial JavaScript loading, and analyzing JavaScript bundles, developers can significantly enhance the speed and efficiency of their Next.js applications. It's worth noting that while these optimizations primarily apply to the pages router, the insights gained can also be applied to the app router, ensuring comprehensive performance enhancements across the entire application. Through diligent optimization efforts, developers can create Next.js applications that not only load quickly but also deliver an exceptional user experience.