WordPress Embed Optimizer: Fixing Wasteful `preconnect` Links

Hey guys! Let's dive into a fascinating discussion about the WordPress Embed Optimizer and a quirky issue it has with adding unnecessary preconnect links. This article breaks down the problem, explains why it's happening, and explores a potential solution to boost your website's performance. So, buckle up and let's get started!

The Issue: Wasteful preconnect Links

So, here's the deal. The Embed Optimizer in WordPress, a tool designed to enhance performance, seems to be a bit overzealous when it comes to adding <link rel="preconnect"> tags. Specifically, it's adding these tags for resources used in embeds that appear in the initial viewport, even when only dns-prefetch would suffice. This might sound like a minor detail, but it can actually impact your site's loading time and overall efficiency.

The problem was initially brought up in this GitHub issue. The goal was to improve performance by adding preconnect links for resources within initial-viewport embeds. You can see the logic behind this in the Embed Optimizer's code, specifically in the class-embed-optimizer-tag-visitor.php file. Check out this link to see the code in action.

Why preconnect Can Be Wasteful in This Context

The main reason this becomes wasteful is due to the nature of how embeds work. Typically, these resources are loaded within an <iframe>. Because of cross-origin isolation, the preconnect directive often ends up being redundant. The iframed third-party embed essentially has to establish a new connection anyway, making the initial preconnect somewhat pointless.

Imagine you're trying to get to a friend's house within a gated community. Adding preconnect is like pre-warming your car before you even know if the gate will open. You still need to go through the gate (the iframe), and that requires a whole new process. So, the pre-warming might not save you much time in the end.

The Silver Lining: DNS Cache Warming

Now, it's not all doom and gloom. There's a tiny silver lining here. Preconnecting does offer a minor advantage: it warms the operating system's DNS cache. Think of the DNS cache as your phone's contact list. It stores the addresses (IP addresses) of websites, so your browser doesn't have to look them up every time. Since preconnect also includes a dns-prefetch, this explains why some performance benefits were observed when these links were added. It's like having a quick peek at your contact list before making the call.

However, the consensus is that we could achieve even better performance by focusing solely on dns-prefetch in this specific scenario. It's more efficient to just warm the DNS cache without attempting a full connection that will likely be re-established by the iframe anyway.

This very topic was actively discussed among web performance experts on the Web Performance Slack channel. They weighed the pros and cons, ultimately leaning towards the idea that dns-prefetch would be the more optimal solution.

Diving Deeper: Understanding preconnect and dns-prefetch

To really grasp the issue, let's quickly recap what preconnect and dns-prefetch actually do. These are both resource hints, meaning they tell the browser to perform certain actions in the background to speed up page loading.

preconnect

The <link rel="preconnect"> hint tells the browser to establish a connection to a specific domain before the browser actually needs to request a resource from that domain. This involves performing a DNS lookup, establishing a TCP connection, and even negotiating a TLS handshake. It's like making a reservation at a restaurant before you arrive, so you can be seated faster.

By pre-establishing the connection, the browser can save valuable time when it eventually needs to fetch resources from that domain. This is particularly useful for critical resources like fonts or APIs.

dns-prefetch

The <link rel="dns-prefetch"> hint, on the other hand, is a more lightweight approach. It only instructs the browser to perform a DNS lookup for a specific domain. This resolves the domain name to an IP address and stores it in the DNS cache. It's like checking the restaurant's address before you leave home, so you know where to go.

While dns-prefetch doesn't establish a full connection, it still provides a performance boost by reducing the latency associated with DNS resolution. This can be especially helpful for third-party resources where the connection establishment might not be as critical as the DNS lookup itself.

Why dns-prefetch Might Be the Better Choice Here

So, why are we suggesting dns-prefetch over preconnect in this particular scenario with iframed embeds? Here's a breakdown of the reasoning:

1. Cross-Origin Isolation: As mentioned earlier, iframed embeds often operate in a different security context (cross-origin). This means that the browser might need to re-establish a connection even if one was pre-established using preconnect. It's like having a reservation under a slightly different name – the restaurant might still need to verify your identity.
2. Reduced Overhead: dns-prefetch is a less resource-intensive operation than preconnect. It only performs a DNS lookup, whereas preconnect sets up a full connection. This lower overhead can be beneficial, especially on mobile devices or networks with limited bandwidth.
3. Specificity: In the case of embeds, the primary bottleneck is often the DNS lookup. The actual connection establishment is less of a concern because the iframe will handle that separately. Therefore, focusing on dns-prefetch directly addresses the main performance hurdle.

In essence, using preconnect in this scenario is like using a sledgehammer to crack a nut. It's an overkill solution that consumes more resources than necessary. dns-prefetch, on the other hand, is a more targeted and efficient approach.

The Proposed Solution: Let's Optimize This Thing!

The proposed solution is pretty straightforward: modify the Embed Optimizer to use dns-prefetch instead of preconnect for initial-viewport embeds loaded within iframes. This would eliminate the wasteful connection attempts while still providing the benefit of DNS cache warming.

This change would involve tweaking the code in the class-embed-optimizer-tag-visitor.php file. Specifically, the logic that adds the preconnect links would need to be adjusted to add dns-prefetch links instead.

The Benefits of Switching to dns-prefetch

So, what kind of improvements can we expect from this switch? Here's a quick rundown:

• Reduced Page Load Time: By eliminating unnecessary connection attempts, we can shave off valuable milliseconds from the page load time. This translates to a faster, more responsive website for your users.
• Lower Resource Consumption: dns-prefetch consumes fewer resources than preconnect, which can be especially beneficial for users on mobile devices or metered connections.
• Improved Performance Score: Search engines like Google consider page load time as a ranking factor. By optimizing resource hints, we can improve our website's performance score and potentially boost its search engine ranking.
• Cleaner Code: Using the right tool for the job (in this case, dns-prefetch) results in cleaner, more efficient code. This makes the codebase easier to maintain and debug.

Conclusion: A Small Change, a Big Impact

In conclusion, the WordPress Embed Optimizer's current implementation of preconnect for initial-viewport embeds within iframes is a bit wasteful. While it does provide some benefit through DNS cache warming, a more efficient approach would be to use dns-prefetch instead.

Switching to dns-prefetch can lead to faster page load times, reduced resource consumption, and an overall improved user experience. It's a small change that can have a significant impact on your website's performance.

So, what do you guys think? Are you on board with this optimization? Let's keep the conversation going and work towards a faster, more efficient web! And keep an eye on the GitHub issue for updates on this potential fix.