Should you combine CSS and JavaScript files in 2025?

If you’ve spent any time optimizing your WordPress site, you’ve probably come across the advice:

“Combine your CSS and JS files to improve performance.”

For years, this was golden wisdom. Fewer requests meant faster websites, especially on shared hosting or older browsers. But as with all things on the web, times change.

Thanks to HTTP/2, the once-standard practice of combining files isn’t always the best solution. In fact, on many modern sites, it can do more harm than good.

So the big question in 2025 is:

“Should you still be combining your CSS and JavaScript files?”

Let’s break it down.

Under HTTP/1.1, browsers had a serious limitation: they could only open a few connections per domain (usually 6). That means when your site loads:

• CSS file #1 loads…
• Then CSS file #2…
• Then JavaScript #1…
• And so on.

This creates a waterfall effect, one file loads after another, with pauses in between. Too many requests? Your site slows to a crawl.

So developers got smart. They started combining multiple CSS and JS files into one. One big file = fewer requests = faster site.

Simple. Effective. Necessary.

Enter HTTP/2, the upgraded version of the HTTP protocol used by almost all modern browsers and hosts in 2025.

Here’s what HTTP/2 brings to the table:

• Multiplexing: Multiple files load at the same time over a single connection.
• Header compression: Reduces overhead on each request.
• Prioritization: Important assets can load first.

In short: HTTP/2 removes the penalty for multiple requests.

So now, instead of combining everything into one massive CSS or JS file, you can serve them separately, and your browser will download them all simultaneously.

Even with HTTP/2, combining files isn’t dead. There are specific scenarios where it can still provide performance benefits:

The key takeaway? Don’t blindly combine. Test. Optimize. Repeat.

Check your site’s HTTP version using the free KeyCDN HTTP/2 Test before you make any changes.

Just enter your domain and click “Test”. If it says “Yes” for HTTP/2 support — great! You’re ready for next-gen optimization.

If not? You might want to consider switching to a host that supports HTTP/2 or enabling it via your CDN or server settings.

In this example, we’ll be using the WP-Optimize plugin to manage CSS and JavaScript files. WP-Optimize makes it easy to combine, minify, defer, or leave your files untouched, giving you full control over your site’s performance settings.

1. Go to your WordPress dashboard.
2. Navigate to WP-Optimize → Minify.
3. Enable Minify

4. Under the “JavaScript” and “CSS” tabs, enable merging of files

5. Click “Save settings” and clear your cache.

1. Same path: WP-Optimize → Minify
2. Uncheck the enable merging of files option under each tab.
3. Save and purge your cache.

Once you’ve enabled or disabled file combining, test your site using tools like:

GTmetrix
PageSpeed Insights
WebPageTest

Look at:

• Total load time
• Number of HTTP requests
• Time to First Byte (TTFB)
• Largest Contentful Paint (LCP)

Compare both versions (combined vs uncombined). You’ll often find that uncombined, minified files perform better on HTTP/2, while combining helps on HTTP/1.1 or file-heavy setups.

• Always minify, even if you don’t combine. It’s lightweight and improves performance across the board.
• Use defer or async loading for JavaScript when possible (also in the Minify settings).
• Monitor your site’s behaviour, combining files can occasionally cause layout or JS conflicts.

Maybe. Maybe not.

The best performance setup is contextual, based on your site, hosting, and audience.

Thankfully, with tools like WP-Optimize, you don’t have to guess. You can toggle settings, test performance, and make data-driven decisions.

Combining files may be old-school, but it’s not obsolete. It’s strategic.

WP-Optimize gives you the power to manage this with a few clicks.
Head over to WP-Optimize → Minify and try toggling file combining today.

And don’t forget to test your results, because when it comes to performance, the numbers never lie.