Speculation Rules API: Mastering Predictive Prefetching and Cache TTLs

When I first started playing with the Speculation Rules API: Architecting Instant Navigation Strategies, the "instant" feel was addictive. Seeing a page load in roughly 50ms because the browser had already fetched it felt like magic. But that magic comes with a catch: if your cache strategy isn't aligned with your data volatility, you’re just serving users a fast, outdated experience.

Predictive prefetching is a powerful lever, but it’s easy to pull it too hard. If you prefetch a personalized dashboard or a frequently updating news feed, you’re essentially banking on the idea that the data won't change between the prefetch and the actual click.

The Reality of Speculation Rules API and Cache TTL

The browser’s prefetch cache is just another cache. If you send a Cache-Control: max-age=3600 header, that browser is going to hold onto that prefetched document for an hour. If the user clicks the link 30 minutes later, they get the hour-old version.

We ran into this while working on a project where we used the API to prefetch product category pages. We saw a nice bump in our Core Web Vitals, but we started getting support tickets about pricing discrepancies. The prefetch was grabbing the price from 20 minutes ago, while the actual database had updated.

Here is how we broke down the cache logic:

Managing Data Freshness

To make this work, you have to treat the Speculation Rules API as a hint, not a command. The browser is going to execute the fetch based on the rules you define, but the network layer still honors standard HTTP caching.

We first tried disabling caching entirely for prefetched routes. That was a mistake. It defeated the purpose of the Fetch Priority API improvements because every navigation became a full network round-trip. Instead, we shifted to a stale-while-revalidate approach.

By setting Cache-Control: public, max-age=0, must-revalidate, stale-while-revalidate=60, we instructed the browser to:

1. Use the prefetched content if it's there.
2. Immediately trigger a background revalidation if the content is "stale."
3. Update the cache for the next navigation without blocking the current one.

This is a massive win for Data Hydration Strategies. You get the speed of the prefetch, but the user eventually sees the fresh data if they stick around long enough.

Flow diagram: User Hovers Link → Speculation Rules; Speculation Rules → Browser Fetches Page; Browser Fetches Page → Cache Store; Cache Store → User Clicks Link; User Clicks Link → Check TTL; Check TTL → Fresh Instant Load; Check TTL → Stale Serve Stale + Revalidate

Why You Shouldn't Prefetch Everything

It’s tempting to add a broad rule that prefetches every link on the page. Don't do it. You’ll hit the browser’s internal limits, and more importantly, you’ll waste your users' data plans and bandwidth.

I recommend starting with your "Top 5" high-conversion paths. Use the Server-Timing API for INP Optimization to identify which pages are slow to respond on the backend. If a page takes 400ms to generate, that’s your prime candidate for prefetching. If it’s already sub-100ms, the performance gain of prefetching might not justify the complexity of managing the cache.

A Caveat on Implementation

One thing I’m still tracking is how different browsers handle the prefetch cache eviction. Chrome is generally aggressive, but it’s not infinite. If you have a massive site, the browser will eventually drop old prefetched items to save memory.

Don't assume that just because you prefetched it, it’ll be there. Always design your application to handle a fallback. If the network is down or the cache is cleared, the page should still load predictably. Never rely on the Speculation Rules API for core functionality—it’s a performance enhancement, not a data-fetching architecture.

Next time, I’d probably spend more time testing the impact on battery life for mobile devices. Prefetching isn't free; it consumes radio power and CPU cycles. We’re currently monitoring navigator.connection.saveData to disable speculative loading for users on limited data plans, which seems like a sensible middle ground.