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Are JavaScript libraries really that bad for performance?

Earlier this week, I posted a video on YouTube about how to create your own personal jQuery, and wrote a text article version of it as well.

On YouTube, someone in the comments was very insistent that jQuery has no impact on performance whatsoever.

What is it, 30kb? Even at 7Mps it still downloads in just a few milliseconds. You don’t need the latest hardware or superfast broadband to handle it. And it could already be cached in the client. Optimizing images will gain far more performance than avoiding a jQuery download.

Today, I wanted to explain why this is wrong.

Let’s dig in!

A byte of JavaScript isn’t like a byte of other code

Because it’s a scripting language, browsers handle JavaScript differently.

Optimizing images is important, but will generally not get the same performance gains that you’ll see from optimizing JavaScript.

JavaScript isn’t run immediately.

When a browser downloads an image file, it can start rendering it right away, before it’s even finished downloading the whole thing.

With a JavaScript file, the browser waits for the whole thing to download, then it compiles it, then it reads through the whole file top-to-bottom to see what’s in it. If there are ES imports, those need to get downloaded, compiled, and read, too.

Then the JavaScript can finally, actually run.

JavaScript is render blocking. Images are not.

While all of that stuff I just described is happening, the browser stops rendering the UI until it’s finished. Because JavaScript often modifies the DOM, browsers stop rendering anything until they know what the JS file is going to do.

This helps them avoid painting things that are just going to be removed by JavaScript anyways.

All of this is to say, 1 byte of JS is much more expensive than a byte of JPG or PNG.

So yea, optimize your images, but know that JavaScript is still much worse for performance than images or HTML or CSS are.

If you want to learn more about how this works under the hood, Addy Osmani from Google wrote a detailed post about it.

The actual size of jQuery is much bigger than 30kb

That’s the minified and gzipped size. Once browsers unzip jQuery, it’s actually 285kb, more than nine times larger.

That’s true for all major libraries, by the way.

Those gzipped sizes are super important for transit over the wire and downloading times. But once the file is actually compiled, the bigger “real size” matters more.

More JavaScript means more memory usage and more abstraction and slower processing times.

Actual data

Let’s look at some data.

WordPress found that jQuery in themes is one of the single biggest performance bottlenecks for WordPress sites.

jQuery is the most common JavaScript-based performance problem in themes.

I ran an analysis using the PageSpeed Insights API, gathering performance reports for the most popular 100 WordPress themes according to the Themes API. The first interesting aspect to look at was finding out how many themes of those even use jQuery and whether it is in fact as widely used as believed. And yes, the reports flagged jQuery as by far the most notable JavaScript file that leads to performance issues, in 76 of the 100 themes. The table below shows the most flagged WordPress core JavaScript files commonly loaded by themes.

A few years back, the team removed jQuery from their site and found substantial performance gains, particularly for folks in the P95 percentile (the 5 percent of users with the slowest connection).

We see many of our key metrics trending down (for p75) after the change, including frontend time, First CPU Idle, JS Long Tasks.

If we compare before / after for our JavaScript performance, we can easily see the percentage improvements. If we look at the extreme end (our P95 users). they also see significant improvements.

Click through to Matt Hobbs’ Twitter thread to see the charts and metrics they used.

And then just this week, performance specialist Tim Kadlec tweeted out this

Phew. What a difference a :last makes in jQuery. Third-party @shopify app with a very long list of selectors in a click handler.

With the deprecated :last selector: ~616ms
Without: ~16ms

It’s not just jQuery, of course. From Zach Leatherman…

An analysis of Core Web Vitals across 9.3 million web sites as of February 6, 2023 shows that Core Web Vitals for both React and Next.js shows that both perform worse than the aggregation of all other sites in the archive for both mobile and desktop.

Why are libraries slow?

In a word: abstraction.

Every layer of abstraction you add mores more work browsers, takes up more space in memory, and slows processing times down.

A good example of this is React vs. Preact. My friend Jeremy Wagner did some testing, and found that Preact was faster to both render the initial UI and handle DOM updates.

Why? Preact is closer to the metal and uses far fewer abstractions under-the-hood, despite having a nearly identical API to React proper.