Resource Prefetching May Slow Down Your Website: Best Practices

The potential of resource prefetching

Resource prefetching represents a proactive approach to web optimization that loads assets before users actually request them. This technique allows us to anticipate user behavior and prepare resources in advance, creating faster page transitions and improved user experiences.

When we implement prefetching, we essentially make educated guesses about where users might navigate next. The browser fetches these anticipated resources during idle periods, storing them locally for immediate use when needed. This prefetching strategy can boost web performance when applied thoughtfully.

We can implement prefetching through simple HTML link elements that specify which resources to load ahead of time. These resource hints work alongside other performance techniques like preload directives to create comprehensive loading strategies.

Loading sequence prioritization

Prefetched assets operate at the lowest priority level within the browser's request queue. This ensures that current page resources receive full bandwidth allocation while prefetched content loads quietly in background processes.

The browser manages these requests intelligently, only fetching prefetched resources when network capacity allows. This prevents performance degradation on slower connections while still preparing future resources.

Early prefetch requests causing bandwidth competition

When we analyzed request waterfalls on our websites, we discovered that prefetch requests frequently initiated too early in the loading process. These resource prefetch requests can slow down websites by creating unexpected bandwidth conflicts with critical page resources.

Despite browsers assigning lowest priority to prefetched content, our high-priority Largest Contentful Paint images experienced delayed download times. This demonstrates how preloading mechanisms can inadvertently impact website performance metrics even with proper priority scheduling in place.

Do Performance Tests Reflect Actual User Experiences?

Laboratory testing environments create artificial conditions that rarely mirror authentic browsing scenarios. Most website visitors operate with significantly faster internet connections and greater bandwidth availability compared to the constrained conditions typically used in automated testing tools like Lighthouse.

To understand genuine user impact, we implemented real user monitoring to measure actual loading behaviors across different connection types and device capabilities. Our tracking focused specifically on image load completion versus prefetched resource delivery timing.

The monitoring revealed unexpected patterns in how resources actually load for real users:

• Prefetched JavaScript files frequently completed downloading before images finished loading
• Page load times varied significantly from laboratory predictions
• Core Web Vitals measurements showed different patterns in live environments
• User experience metrics differed substantially from synthetic test results

Network conditions play a crucial role in these variations. JavaScript resources loaded from the same domain benefit from existing server connections, while images often require establishing new connections. This connection advantage frequently allows prefetched scripts to complete faster than primary page images, directly impacting FID measurements and overall web performance in ways that laboratory conditions cannot accurately simulate.

Can Turning Off Prefetching Boost Site Speed?

We conducted testing by serving prefetch link tags to only 50% of visitors to measure performance differences. The results showed minimal overall impact on loading times.

Key Performance Findings:

• 75th percentile: 100ms faster LCP without prefetching
• 90th percentile: Similar 100ms improvement when disabled
• 95th percentile: Prefetching actually performed better

When we analyze critical assets and their loading patterns, the data reveals mixed results across different user experiences. Performance monitoring tools can help identify whether prefetching helps or hinders your specific site configuration.

The inconsistent results suggest that prefetching benefits vary significantly based on user connection speeds and device capabilities.

Do Prefetched Resources Typically Begin Loading Before Idle Time?

Many websites initiate prefetching much earlier than intended, often before the browser reaches an idle state. We observed that prefetched resources should load at the lowest priority when the browser has spare capacity, yet this theoretical behavior doesn't always match reality.

Real-world prefetching patterns reveal:

• Resources begin downloading within seconds of page load
• Lowest-priority JavaScript files start loading before main content completes
• Some sites prefetch over one megabyte of data before key performance milestones

While prefetching can improve performance for future navigations, the early loading behavior contradicts the expected idle-time implementation. This premature prefetching doesn't necessarily harm Largest Contentful Paint metrics, but it demonstrates a gap between theory and practice.

We found multiple examples where prefetched content began loading well before the primary page finished rendering. This suggests that browsers may interpret "idle time" differently than developers expect, or that other factors trigger prefetch requests sooner than optimal.

Avoiding Performance Issues with Resource Prefetching

We should avoid placing prefetch directives directly in our initial HTML markup or HTTP headers. This prevents prefetched resources from competing with critical content during the initial page load.

Dynamic Implementation Approach

We can implement prefetching through JavaScript after the page loads completely:

window.addEventListener("load", () => {
  const link = document.createElement("link");
  link.as = "style";
  link.rel = "prefetch";
  link.href = "next-page.css";
  document.head.appendChild(link);
});

Key Timing Considerations

• Wait for the load event before adding prefetch links
• Use the defer attribute for related scripts to maintain proper execution order
• Prioritize current page resources over future page assets

This approach ensures our main content loads efficiently while still preparing resources for subsequent navigations.

Final Thoughts

We observe that early resource loading can create competition with critical page elements during initial rendering. Most websites won't experience significant slowdowns since bandwidth limitations rarely constrain modern web performance.

Browser implementations vary in their approach to resource timing. Chrome loads prefetched content immediately, while Firefox delays these requests until primary content renders. This difference affects how prefetching impacts user experience across different browsers.

Key considerations for implementation:

• Monitor bandwidth usage on mobile connections
• Test across multiple browser environments
• Prioritize critical rendering paths over speculative loading

We recommend strategic application rather than aggressive prefetching to maintain optimal performance.