Key Takeaways

• Ad prefetching retrieves and prepares ads before they're needed, reducing latency by 30-50% in most implementations

• Primarily an in-app technique, prefetching works for interstitial, rewarded, banner, and video ad formats

• Prefetched ads can be held in cache for up to 60 minutes according to Google's policies

• While powerful, this technique increases requests to demand partners and requires careful resource management

What Is Ad Prefetching and Why Should You Care?

Ever notice how some mobile games show ads almost instantly between levels? Or how certain streaming apps manage to deliver video ads without that awkward buffering pause? That's likely ad prefetching in action - a technique that's become increasingly important in the monetization toolkit.

In its simplest form, ad prefetching is the process of proactively loading and preparing ads before they're actually needed. Instead of waiting until the exact moment an ad needs to be displayed, the system requests it in advance, stores it temporarily, and has it ready to go when the user reaches an ad trigger point.

Think of it like meal prepping for the week – rather than cooking from scratch every night when you're already hungry and tired, you've got something ready to heat up and serve immediately.

How Ad Prefetching Actually Works

The typical prefetching process follows these steps:

1. Anticipation: The system identifies likely upcoming ad opportunities (level completion in a game, natural breaks in content, etc.)

2. Proactive requests: Ad requests are sent to demand sources before the ad slot is actually visible

3. Temporary storage: Retrieved ads are cached locally, ready for immediate display

4. Just-in-time delivery: When the user reaches the trigger point, the pre-loaded ad appears instantly

This whole flow is particularly effective in mobile apps, where developers have greater control over the environment compared to websites. While some web implementations exist, prefetching shines brightest in native app environments.

Multiple Ad Formats, Multiple Benefits

A common misconception is that prefetching only matters for video streaming. In reality, it's valuable across several formats:

Interstitial Ads

These full-screen ads that appear at natural transition points benefit enormously from prefetching. Without it, users might face a noticeable delay after completing a level or article, creating friction in the experience. With prefetching, the interstitial appears instantly, maintaining user flow.

Rewarded Video Ads

When a user opts in to watch an ad for a reward, any delay creates frustration. Prefetching ensures the video starts immediately after the user agrees, significantly improving the perception of the exchange.

Banner Ads

Even simple banner ads can benefit from prefetching, particularly when transitioning between sections of an app. Having the next set of banners ready to display eliminates blank spaces during navigation.

In-Stream Video

For streaming content, prefetching helps solve the infamous "buffering" problem during ad breaks, creating smoother transitions between content and commercials.

Server-Side vs. Client-Side Implementation

Prefetching can be implemented in two primary ways, each with distinct advantages:

Client-Side Prefetching

This happens directly on the user's device. The app or browser proactively requests ads and stores them locally. This approach:

• Utilizes the user's device resources (memory, processing power)

• Can leverage specific user data for targeting

• Works particularly well for predictable ad placements

• May consume additional battery and data on mobile devices

Server-Side Prefetching

Here, a server component (like a SSAI solution) handles the prefetching. This method:

• Reduces load on the user's device

• Centralizes the prefetching logic

• Is particularly valuable for video streaming

• Requires more complex backend implementation

Research from Microsoft found that well-implemented client-side prefetching in mobile apps can actually reduce overall energy consumption by up to 50%, despite the additional requests, by eliminating inefficient last-minute processing.

The 60-Minute Window: Google's Cache Policy

An important constraint to be aware of: according to Google's policies, prefetched ads can only be held in cache for a maximum of 60 minutes before they must either be displayed or discarded. This limitation helps ensure:

1. Ads remain fresh and relevant when shown

2. Advertisers get fair exposure for time-sensitive campaigns

3. Impression metrics stay accurate

This time window means your prefetching strategy needs to be calibrated to the typical user session length in your app. Prefetching too far ahead risks wasting resources on ads that will expire before they can be shown.

Benefits That Actually Matter

While the technical aspects are interesting, let's talk about what really matters: the tangible benefits.

Dramatically Improved User Experience

The most immediate impact is eliminating those awkward pauses before ads appear. This might seem counterintuitive – after all, don't publishers want users to see ads? – but a smoother experience keeps users engaged with your content longer, creating more overall ad opportunities.

Higher Fill Rates

When ad requests happen at the last possible moment, any delay or timeout means a missed opportunity. Prefetching gives the ad system more time to complete the request, significantly improving fill rates. This is particularly valuable during high-traffic periods when ad servers might be under heavy load.

Better Performance Metrics

Ads that load instantly tend to perform better across key metrics:

• Higher viewability (they appear precisely when they should)

• Better completion rates for video ads (fewer abandons due to buffering)

• Improved overall engagement

The Hidden Costs: What to Watch Out For

Prefetching isn't without tradeoffs. Here are some challenges to consider:

Increased Requests to Demand Partners

This is the big one many publishers overlook. Prefetching inherently means you'll be making more ad requests than will actually be shown. If you prefetch ads for every potential placement but the user ends their session early, those requests were essentially "wasted."

Some demand partners have:

• Request caps or throttling mechanisms

• Pricing models that factor in request volume

• Analytics that might be skewed by high request-to-impression ratios

Always discuss your prefetching strategy with key demand partners to ensure alignment.

Resource Management Challenges

Prefetching consumes device resources: memory, processing power, battery, and potentially data. Finding the right balance requires testing and optimization.

A study published on ResearchGate noted that overly aggressive prefetching in mobile apps can actually negate the performance benefits if not carefully tuned.

Cache Expiration Complexity

With Google's 60-minute cache limitation, you'll need systems to track the age of prefetched ads and request fresh ones when needed. This adds another layer of complexity to your implementation.

Implementation Best Practices

If you're considering implementing prefetching, here are some practical tips:

1. Start with predictable placements – Interstitials after level completion or natural content breaks are ideal candidates

2. Implement intelligent triggers – Use user behavior patterns to predict when ads will likely be needed

3. Respect resource constraints – Consider device type, connection quality, and battery level when deciding how aggressively to prefetch

4. Monitor demand partner relationships – Keep an eye on request volumes and discuss your strategy with key partners

5. Implement fallback mechanisms – Have a plan for when prefetched ads expire or aren't available

Is Ad Prefetching Right for Your App?

Prefetching makes the most sense for:

• Mobile games with predictable level completion or break points

• Content apps with clear section transitions

• Video streaming apps with pre-determined ad break timing

• Apps with longer average session lengths (but still under 60 minutes)

• Publishers with strong technical resources for implementation and optimization

If your app has unpredictable user flows or very short session times, the benefits might not justify the implementation complexity.

The Future of Ad Prefetching

As machine learning advances, we're seeing the emergence of predictive prefetching systems that can analyze user behavior patterns and prefetch only the ads most likely to be needed. Combined with edge computing improvements, this promises even better performance with fewer wasted resources.

Some platforms are also experimenting with longer cache windows for certain ad types, which could make prefetching viable for a broader range of applications.

Final Thoughts

Ad prefetching represents one of those rare technical optimizations that benefits everyone in the ecosystem: publishers get better fill rates and revenue, advertisers get better performance and viewability, and users get a smoother experience.

While it requires careful implementation and monitoring, particularly regarding demand partner relationships and resource management, the potential upside makes it worth exploring for most app developers serious about monetization.