Google Cloud CDN cache-fill latency caused first-request slowdowns and the origin warm-up strategy that smoothed spikes

Published On - November 18, 2025

Sophia Willson blog

Imagine this: your website is perfectly coded, hosted on a reliable server, and your content is top-notch. You expect smooth sailing. But when a user visits a page for the first time, they get… a long wait. Why? Because of something called cache-fill latency in Google Cloud CDN.

TL;DR:
When a user requests content that isn’t already cached in Google Cloud CDN, the CDN has to fetch it from your origin server. This initial fetch, called a cache-fill, can be slow and cause sudden latency spikes. If many users hit uncached content at once, the server can become overwhelmed. A smart solution is called the origin warm-up strategy, which pre-loads popular paths into the cache before users even ask for them.

What Is Google Cloud CDN?

Google Cloud CDN (Content Delivery Network) helps make websites faster. It stores copies of your web pages at locations all over the world. So when someone wants to see your site, it loads from the closest location, not your main server.

This is great for speed, but there’s a catch. If the page hasn’t been accessed recently, it might not be in nearby caches. That’s when cache-fill latency kicks in.

Understanding Cache-Fill Latency

Let’s say a user in Paris tries to load a blog post. If the Paris CDN node doesn’t already have the content cached, it sends a request to your main server — maybe in Iowa. That back-and-forth request takes time. We call that delay cache-fill latency.

This can be particularly frustrating when:

  • A new blog post is posted and is trending.
  • A live stream or new product page goes live.
  • Seasonal traffic causes spikes, like during holidays.

So, even though your website might normally load in under a second, the first visitor may experience 3… 5… maybe 10 seconds of delay. Not fun.

Why First-Request Slowdowns Happen

Here’s what’s going on under the hood:

  1. A user requests a page.
  2. The edge location checks its cache for that content. It’s missing.
  3. The CDN sends a fetch request to the origin server.
  4. The server processes the request, handles database calls, renders the page, and sends it back.
  5. The CDN receives the data, stores it, and sends it to the user.
  6. Next time someone requests the page, it’s lightning-fast… but that first user? Not so much.

Multiply this effect across thousands of users suddenly visiting different uncached pages, and your origin server starts sweating.

The Danger of Thundering Herds

When a large user base bombards a server with requests for content not in cache, we call this a “thundering herd.” The origin server must answer every one, sometimes for the same data over and over. This causes:

  • Slower response times
  • Higher server costs
  • Possible server crashes

It’s ironic. The very system built to boost performance ends up causing slowdowns — but only during those first moments.

What’s the Fix? Meet the Origin Warm-Up Strategy

This is where the origin warm-up strategy shines. It’s like preheating your oven before baking cookies. Before users arrive, you pre-fetch content into the CDN so it’s ready and warm.

Here’s how it works:

  1. List the paths or pages you expect users to hit soon.
  2. Write a simple script that sends HEAD or GET requests to those URLs periodically.
  3. The CDN thinks someone requested that content and fills the cache with it.

Voila! When real users show up, their content is already cached, with no slow cache-fill delay.

Types of Warm-Ups

Several strategies can be combined for even better results:

  • Scheduled Warm-Ups: Run scripts to pre-populate caches daily or hourly, depending on traffic.
  • Trigger-Based Warm-Ups: When a new article or product goes live, auto-send requests to cache it immediately.
  • Geo-specific Warm-Ups: Target specific regions with warm-ups based on time zones or events.
  • Popularity-Driven Warm-Ups: Focus on URLs with high historical traffic — blog posts, landing pages, etc.

These warm-ups don’t need real users. Just fake request “pings” are enough to let Google Cloud CDN prepare your data.

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Smart Tips for Smooth Cache-Fills

Want to go beyond simple scripting? Here are some nifty best practices:

  • Use cache-control headers: Make sure content is marked to be cached correctly. No-cache headers defeat the purpose.
  • Compress content: Sends smaller payloads into the CDN quicker.
  • Group requests: You can “bundle” warm-up requests into a few batch calls to reduce server strain.
  • Monitor with logs: Use Cloud Logging to detect slowouts and cache misses so you can react fast.

Real-World Example: The Flash Sale Fiasco

A popular eCommerce site ran a surprise flash sale. Traffic surged. Shoppers flooded to pages that weren’t warmed up. The result?

  • 10-second page loads
  • Cart API timeouts
  • Lost revenue

Afterward, they implemented an origin warm-up strategy. Before each future sale, they pinged hot pages and loaded images to the edge cache. Performance improved dramatically. Customers were happy and so was the revenue graph.

Automating the Warm-Up Process

You don’t have to manually select pages or run crontabs. Here’s what automation could look like:

  1. Analyze Google Analytics to identify top visited pages.
  2. Feed those URLs into a cloud function or script runner.
  3. Execute warm-up requests right before peak traffic hours.

Optional bonus: add machine learning to predict popular content spikes before they happen!

Cache-Tunable Settings in GCP

Google Cloud gives you levers to play with:

  • Time to Live (TTL): Set how long cached data stays fresh. Too short? You get more cache-misses. Too long? Might deliver stale data.
  • Negative caching: Even 404s can be cached to save on future origin calls.
  • Custom policies: Set different rules for media, static pages, and dynamic parts of your site.

Mastering these lets you control when to refetch from origin or serve from cache without guesswork.

Conclusion: Keeping Things Fast from the Start

Google Cloud CDN is powerful, but it has a weak spot: those first requests that trigger a cache-fill. The result? Slow responses right when your users are most excited to see what you’ve launched.

Luckily, the origin warm-up strategy is simple, cost-effective, and easy to implement. Whether you use scheduled scripts, cloud triggers, or full analytics-driven automation — you’ll smooth out spikes and keep performance crisp.

Your users may never notice the strategy behind the speed. But they’ll stay longer, click more, and bounce less. And your servers? They’ll thank you with fewer errors and more uptime.

Next Steps

  • Start with a small warm-up script on your most visited pages.
  • Use logs to measure improvement in response times.
  • Iterate and expand your cache pre-loading efforts!

With warm-ups, your first user isn’t a test subject—they’re just another happy visitor.