I made my site faster in 2 minutes with one Chrome file

I was reading up on the Chrome Private Prefetch Proxy feature, and the moment I understood what it does, I jumped into my server logs. I wanted to see if Chrome had been hitting /.well-known/traffic-advice on my site and getting nothing back.

And guess what. It had.

A steady drip of 404s on that exact URL. Chrome was politely asking my server, “Hey, do you want me to preload your pages for the next visitor coming from search?” and my server was answering, “file not found, go away.” Every single time.

So I fixed it in under 2 minutes. Here is exactly what I did, why it matters, and how you can do the same on WordPress.

What is /.well-known/traffic-advice actually doing?

Since Chrome 103 (which shipped in 2022), Chrome has run a feature called the Private Prefetch Proxy. When someone is on Google Search and is about to click through to a result, Chrome can quietly preload that page through a privacy proxy. By the time the user clicks, the page is already partly loaded.

Google’s own data on this puts the LCP improvement at 20 to 30 percent on prefetched pages. LCP is a Core Web Vitals ranking factor. So this is a free Core Web Vitals improvement for new visitors arriving from search. That is the highest value traffic on most sites.

The catch: Chrome only does this if your site says yes. The way you say yes is by serving a tiny JSON file at exactly this path:

/.well-known/traffic-advice

If the file is missing, Chrome logs a 404 and moves on. No prefetch.

What goes in the file

Two valid formats. To opt in fully:

[{"user_agent": "prefetch-proxy", "fraction": 1.0}]

fraction: 1.0 means “you can prefetch 100 percent of incoming requests.” You can dial it down (0.5 = 50 percent) if you are worried about origin load, but for almost every site, 1.0 is correct.

To opt out completely:

[{"user_agent": "prefetch-proxy", "disallow": true}]

One thing that matters more than people think: the response has to be served with a specific MIME type or Chrome will ignore it.

Content-Type: application/trafficadvice+json

Plain application/json does not work. Plain text does not work. It has to be exactly that.

How I added it to WordPress

There are two clean ways to do this on a WordPress site.

Method 1: Add it via your theme’s functions.php

This is the option I used. It works regardless of file system access, and it stays in the theme, so it follows the site if you migrate hosts.

Drop this in your child theme functions.php:

add_action('init', function () {
    add_rewrite_rule(
        '^\.well-known/traffic-advice$',
        'index.php?traffic_advice=1',
        'top'
    );
});

add_filter('query_vars', function ($vars) {
    $vars[] = 'traffic_advice';
    return $vars;
});

add_action('template_redirect', function () {
    if (intval(get_query_var('traffic_advice')) !== 1) return;
    header('Content-Type: application/trafficadvice+json');
    header('Cache-Control: public, max-age=86400');
    echo '[{"user_agent":"prefetch-proxy","fraction":1.0}]';
    exit;
});

After dropping that in, go to Settings → Permalinks and click Save Changes. That flushes the rewrite rules. Done.

Method 2: Static file (if you have file system access)

Create a file at the root of your site:

/.well-known/traffic-advice

Content:

[{"user_agent":"prefetch-proxy","fraction":1.0}]

Then make sure your server returns the right MIME type. On Apache, add this to your .htaccess:

<Files "traffic-advice">
    ForceType application/trafficadvice+json
</Files>

On Nginx:

location = /.well-known/traffic-advice {
    default_type application/trafficadvice+json;
    add_header Cache-Control "public, max-age=86400";
}

A note on Cloudflare

If you run Cloudflare in front of WordPress (I do), the static file approach works without anything special, because Cloudflare just passes the path through to the origin. If you want to skip the origin entirely, you can ship the JSON from a Cloudflare Worker without touching your server.

How to verify it is working

Open your terminal and curl the URL:

curl -i https://yourdomain.com/.well-known/traffic-advice

You should see:

HTTP/1.1 200 OK
Content-Type: application/trafficadvice+json

[{"user_agent":"prefetch-proxy","fraction":1.0}]

If you see 404, the rewrite did not catch the URL. Try saving permalinks again. If you see the JSON, but Content-Type: text/html If the MIME type is wrong, Chrome will still ignore it.

Why is this worth doing today?

LCP improvement of 20 to 30 percent on the highest intent traffic on your site, for the cost of writing one rewrite rule. Almost every site I have looked at has the same 404 sitting in its logs right now, going unnoticed.

Check your server logs. If you see Chrome hitting /.well-known/traffic-advice and getting a 404, you are leaving free Core Web Vitals improvement on the table.

Add the file. Verify the MIME type. Ship it.

I will be watching my own LCP in PageSpeed Insights over the next few weeks and will update this post with the actual delta. If you implement it, send me your before-and-after numbers. I want to see them.

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