HTTP/3 vs HTTP/2 Performance: Is the Upgrade Worth It?

While HTTP/2 delivered major performance gains over HTTP/1.1, the differences between HTTP/3 vs HTTP/2 are more subtle.

HTTP/3 doesn't add new functionality to HTTP/2 — instead, it adapts HTTP/2's existing features to work with QUIC, a next-generation transport protocol.

Real-world performance differences between HTTP/3 vs HTTP/2 depend on several factors, such as the geographical distance between client and server, network quality, the number of files the page needs to download, whether it's a first-time visit to the domain, and others.

If you're interested in the technical details, check out our in-depth guide to the HTTP/3 and QUIC protocols — in this article, we'll look into when it's worth adding HTTP/3 support to your website from an SEO and business standpoint.

What is the HTTP Protocol?

HTTP (Hypertext Transfer Protocol) is the network communication protocol that transfers web content over the internet. It consists of a set of rules that define how web servers and browsers communicate.

When a user visits your website, their browser sends an HTTP request to your server, which sends back an HTTP response. This might include the actual web page files or other information such as redirect instructions, error messages, or headers that tell the browser about the server's capabilities (such as HTTP/3 support).

Before requesting a resource, browsers first need to establish a server connection. You can see an example of that in the request waterfalls below.

Why Do HTTP Protocol Versions Matter for Web Performance and SEO?

HTTP currently has three versions in use:

• HTTP/1.1
• HTTP/2
• HTTP/3

The HTTP protocol version used for your website depends on a negotiation between server and browser when a user first visits your domain. Your website will be transferred over the highest HTTP version that both the server and browser support.

For example, if your server supports HTTP/3, your users can receive the website over HTTP/1.1, HTTP/2, or HTTP/3, depending on their browser's capabilities.

For good SEO results, it's important to add support for more advanced HTTP protocol versions to your server (at least HTTP/2, as some servers still only support HTTP/1.1) since better client-server communication leads to faster and more reliable page loads, which translates to better Core Web Vitals scores, higher search engine rankings, and improved user experience.

More advanced HTTP protocol versions can make your website faster in the following ways:

• Faster connection establishment improves Largest Contentful Paint (LCP) scores as browsers can start downloading resources more quickly.
• Simultaneous resource loading (multiplexing) allows browsers to download multiple files concurrently, which improves overall site speed.
• Better error recovery ensures faster page loads when network packets are lost or corrupted during transmission.
• More reliable connections maintain performance even with packet loss or when switching between networks.

The upgrade from HTTP/1.1 to HTTP/2 was significant — the new features HTTP/2 introduced provided a major boost to web performance.

However, HTTP/3's performance benefits are more situational, showing the strongest gains for mobile users on unstable networks, high-latency connections, and returning visitors.

HTTP/3 vs HTTP/2: What Are the Main Performance Differences?

HTTP/3 provides the same features as HTTP/2. The performance differences come from how HTTP messages are packaged and transported across the internet — from the 'how' rather than the 'what'.

While HTTP/2 relies on the TCP protocol for transport, HTTP/3 uses the newer QUIC protocol, which approaches data transmission in a fundamentally different way. The goal of this switch was to make HTTP/2's features work more efficiently and speed up network communication (which sometimes happens, sometimes doesn't under real-world conditions).

HTTP/3's main web performance benefits over HTTP/2 are as follows:

Faster Connection Setup

HTTP/2 requires separate handshakes for transport and authentication, which typically takes 2-3 round trips between server and browser before any actual data can be sent.

HTTP/3 combines these into a single handshake, reducing connection establishment to just one round trip. Plus, for returning visitors, HTTP/3 has 0-RTT (zero round-trip time) resumption, which means that requests can be sent immediately without any handshake delay.

Better Error Recovery

While both HTTP/2 and HTTP/3 use multiplexing (can send multiple files simultaneously), there's an important difference between the two:

• HTTP/2 implements it by itself, as TCP lacks multiplexing capabilities
• HTTP/3 leverages QUIC's multiplexing capabilities

This means that with HTTP/2, when a network packet gets lost, all files stop downloading until the server retransmits the lost packet. This performance issue, called head-of-line (HoL) blocking, can significantly hurt page load times.

QUIC fixes this issue by downloading each file on an independent data stream. For example, if one of your images has a lost network packet, your other resources such as CSS and JavaScript files continue downloading.

Seamless Network Switching

HTTP/2 connections break when users switch networks (e.g. they move from Wi-Fi to a mobile network). The browser must establish a completely new connection, causing delays and interrupting downloads.

HTTP/3 supports connection migration, allowing the same connection to seamlessly continue when switching between networks. This is particularly beneficial for mobile users on the go who frequently move between Wi-Fi and mobile connections.

When Is It Worth Adding HTTP3 Support?

HTTP/3 delivers real performance benefits in the following cases:

• Mobile users on unstable networks
• International audiences
• High-latency connections (e.g. rural connections or large geographic distances)
• Many returning visitors
• Websites with many resources (e.g. eCommerce stores with many product images)

However, HTTP/3's performance benefits will likely be minimal in the following cases:

• Fast, stable broadband connections
• Simple sites with few resources
• Desktop-focused audiences

How to Check What HTTP Version(s) Your Website Is Using

As I mentioned above, your website will use the highest HTTP version that both your server and the respective user's browser support. If you load third-party resources, such as Google Fonts, they can be sent over a different HTTP version than your first-party content.

Basic HTTP/3 Testing

To check which HTTP version(s) your website files are delivered with, you can use our free website speed testing tool.

Once the speed test finishes:

1. Click the Requests tab in the left sidebar to access the request waterfall chart of the page load.
2. Click the Columns option in the top right corner of the page.
3. Toggle on the Protocol column so the HTTP version that each resource used appears on the chart.
4. You can see the HTTP versions in the Protocol column (h2 stands for HTTP/2; h3 stands for HTTP/3).

Note that because of the browser's protocol negotiation process, HTTP/3 typically won't work on the first visit to a domain. This is because most browsers use HTTP/2 first, and only switch to HTTP/3 after the server informs them about its availability.

For example, in the screenshot below, all files were delivered over HTTP/2 (h2):

To test HTTP/3 properly, you need to test the page with a warm cache. To do so:

5. Click the Monitor This Website button in the left sidebar, which allows you to change the test configuration.

Testing HTTP/3 Properly with Warm Load

A warm cache or warm load means the tested URL is loaded twice to see how caching affects web performance on the second load. To test HTTP/3 properly, you need a warm cache because on the second load, the browser and server have already negotiated the HTTP version they'll use for the domain.

After logging into DebugBear, create a new page in the Lab Tests dashboard by clicking the Add Page button at the bottom of the screen.

Once the settings area of the new page opens, add the URL you want to test and configure the test device, test location, and frequency according to your preferences.

Then, click the Show Advanced toggle, open the Warm Load dropdown, and enable both the Disable clearing cache and Warm cache using test URL options:

Once you click the Add Page button, the test automatically starts. When it completes, navigate to the Requests menu to see the new request waterfall chart.

As the below screenshot shows, the HTML page has now been loaded over HTTP/3 (h3):

DebugBear still displays HTTP/2 (h2) for the CSS files because they have been loaded from the browser cache this time.

However, as the browser now knows this domain supports HTTP/3, these CSS files will also be transmitted over HTTP/3 once the file content changes so the browser cache updates or the cache lifetime expires.

HTTP/3 Support

Now, let's see the current state of HTTP/3 support:

Browsers

As of the time of writing, all major browsers support the HTTP/3 protocol.

CDNs

Major content delivery networks (CDNs), including Cloudflare, Akamai, Fastly, and Amazon CloudFront, support HTTP/3 — most of them allow users to simply toggle HTTP/3 on from the settings area.

Web Servers

Most popular web servers, such as Nginx, Caddy, and LiteSpeed, have HTTP/3 modules available.

Apache still lacks native HTTP/3 support with no published roadmap. However, if you use Apache, for example for a WordPress site, you can still enable HTTP/3 by connecting it to a CDN.

Monitor Real User Behavior to Understand Your Audience

To find out whether it's worth adding HTTP/3 support to your server, you'll need to understand your audience to see where they're coming from, what devices and networks they typically use, what main web performance issues they encounter, and more. To do so, you can use DebugBear's real user monitoring (RUM) feature.

If you click the Real User Monitoring > View Count menu, the Overview tab shows the geographic locations of your audience, which can help you decide how much you need to worry about latency:

Using the tabs of the View Count page, you can further drill down into your real user data.

For example, if you click the Breakdown tab, you can see the distribution of devices, browsers, networks, and operating systems across your user base:

You can also view how the different HTTP versions affect Largest Contentful Paint (LCP) for your real users by clicking the Table tab and selecting the HTTP Version option from the Group By dropdown.

In the example below, HTTP/3 users have 13.8% better LCP scores than those served via HTTP/2 (1.44 seconds versus 1.67 seconds) — so for this website, it was worth upgrading to HTTP/3:

To get similar insights, start monitoring your website so that you can decide whether it's worth upgrading to HTTP/3 or another new web technology.

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