App & Browser Testing Made Easy

Give your users a seamless experience by testing on 3000+ real devices and browsers. Don't compromise with emulators and simulators

Get Started free
Home Guide How to Reduce Page Load Time in Javascript

How to Reduce Page Load Time in Javascript

By Shreya Bose, Community Contributor -

As always, the basic question is the best place to start. But before we jump right into it let’s try and understand the importance of page speed score as a website’s speed is one of the most critical factors contributing to the overall experience. To ensure that any website is optimized for intuitiveness and cross-device compatibility, it’s imperative to reduce page load time.

Optimizing website speed and reducing page load times in an evolving search landscape is non-negotiable for any online business. This article will shed more insights on how to reduce website loading time in Javascript.

Why bother with JavaScript Page Load?

Since the internet is basically a fundamental need for the world right now, websites are becoming richer and more complex. To accommodate these trends, developers make increased use of JavaScript frameworks.

While these frameworks do help make websites more appealing and functional, they also require adding more assets (stylesheets, scripts, videos, images). Each of these assets consumes a certain measure of resources on user devices – GPU, CPU, memory, and network.

Obviously, more resources add up to a sluggish, lag-filled, and essentially frustrating user experience. It doesn’t matter how attractive or functional a website is, no user will stick around for a slow website.

Now, exactly how slow JavaScript resources make a website depends on a number of factors: specific hardware and software configurations for devices, in particular. Nonetheless, minimizing the impact of JavaScript should be considered a priority during website design.

How to Reduce Page Load Time in JavaScript?

The two most obvious and effective ways to reduce Javascript page load time:

1. Minify, Compress, Distribute

Remember that internet bandwidth is finite, and latency inevitably occurs when data has to be transferred over long distances. But there are a few ways to reduce this without changing the code.

  • Minify the code with a tool like Closure Compiler, JSMin, or Uglify. Minification reduces the code size without compromising any of its functionality.
  • Compress JavaScript assets in transmission with HTTP transfer encoding such as brotli or gzip.
  • Distribute static assets with a Content Delivery Network (CDN) that can serve website visitors from a place closer to them geographically. This helps reduce the latency that may occur when browsers download website assets.

Note: While these are industry-standard practices, they can be forgotten during software development because the network issues would not be noticeable if the website is being developed on a high-speed connection that most organizations tend to provide for developers.

Of course, it is possible to monitor how a website functions under different network conditions using BrowserStack’s real device cloud. Testers can simulate network conditions on real devices and browsers which means they can run a website on a real device/browser combination, and then change the network strength (from 4G to 2G, for example) and then run the website to check its performance. Now, isn’t that a neat way to handle any of those page load Javascript issues?

Try Testing on Real Device Cloud for Free

2. Parse, Compile, Execute

Most modern browsers’ JavaScript engines run a script in the following phases: parsing, compilation, and execution. A simplified take on these terms is below:

  • Parsing reads the bytes of a script received by the browser and generates an internal representation of the script structure.
  • Compilation takes this internal representation and generates machine code that can be directly run by the processor.
  • Execution is the process of actually running the generated machine code.

Each process consumes processor and memory resources. To optimize these processes to minimize the impact of JavaScript assets, ship less code. Remember, the overall code complexity and structure determine how many resources the processes described above will consume.

Now, as mentioned before, the nature of the user’s device has a significant effect on their experience. Not everyone is accessing the internet with a high-speed connection while using a top-of-the-market MacBook Pro or the latest iPhone. Device power varies immensely, especially with mobile devices.

The code that takes a second to parse, compile and execute on the iPhone X will possibly take 4-5 times longer on a more “average” device. Hence, don’t write code that forces users with low-power devices to wait several seconds while their browser works through megabytes of JavaScript, creating a recipe for bad UX. It has a noticeable effect on reducing JavaScript page load time.

3. Lazy Loading

Use Lazy loading for resources such as images, videos, and other non-critical JavaScript components. Lazy loading defers the loading of these resources until they are needed, reducing the initial page load time.

4. Preloading and Prefetching

Use the preload and prefetch HTML attributes to indicate resources that will be needed in the future, allowing the browser to fetch them in advance during idle time.

How to measure Page Speed of JavaScript on Real Devices

Obviously, developers won’t be expending effort and time on optimizing websites if it isn’t slow and user-unfriendly. On the other hand, if a website is slow and losing users, it must be sped up on priority.

However, instead of waiting for traffic to fall before investigating if page load speed is low, use a tool to test website speed before it is released into production. Additionally, as explained earlier, the same website can load fast or slow on different devices because of the device configuration itself. Therefore, it is mandatory to run a website speed test on a tool that uses real devices.

BrowserStack SpeedLab provides exactly that. When a user enters their URL, this free tool runs a speed test of that website across multiple real browsers, devices, and operating systems. The resulting report will show not just how fast (or slow) the website is, but how it fares in terms of speed across multiple real desktops and mobile devices, as well as multiple browser versions.

BrowserStack Speedlab

Testing Javascript page load on Speedlab

Speedlab - Desktop & Browser Page Resource Summary

With BrowserStack, you also get quick actionable recommendations and insights to improve on. 

Speedlab Page Recommendations

The illustration below showcases how Javascript transfer size can be optimized through Speedlab, thereby reducing page load time.

Javascript page load - Speedlab Performance InsightsSpeed tests run on SpeedLab generate accurate results, every time. Developers and testers can monitor their website status before and after making optimizations, thus making it easier to track progress, streamlining their efforts, and bringing efficacy into the development cycle.

Try SpeedLab for Free

Cross browser testing Manual Testing Website Speed Test

Featured Articles

How to check website loading time

5 Critical Metrics that can help improve Site Speed

App & Browser Testing Made Easy

Seamlessly test across 20,000+ real devices with BrowserStack