When to Cache, When to Compute, When to Preload

This content originally appeared on DEV Community and was authored by DCT Technology Pvt. Ltd.

Every millisecond counts.

Yet developers often get stuck asking the wrong question:

“Should I compute this or cache it?”
Or worse…
“Should I preload everything just to be safe?”

The truth?
There’s no one-size-fits-all. But there is a smart way to decide.

Let’s break down when to cache, when to compute, and when to preload — and how making the wrong choice could cost you speed, scalability, and sanity.
This post is for web devs, backend engineers, frontend ninjas, and anyone who wants performance without the guesswork.

First: Why This Decision Matters

Caching too much = stale data
Computing everything = slow response
Preloading it all = bloated initial load

Performance isn’t just about speed — it’s about balance.

So here’s a real-world guide (with examples, code, and resources) to know exactly what to cache, what to compute, and what to preload.

When to Compute

Compute when the data changes frequently or is personalized per user.

Use cases:

• Real-time dashboards (stock prices, sensor data)
• Dynamic reports
• Anything user-specific (e.g., cart totals)

Example:

function calculateCartTotal(cartItems) {
  return cartItems.reduce((total, item) => total + item.price * item.qty, 0);
}

This shouldn’t be cached — it’s fast, per-user, and always changing.

Pro tip: If a calculation is quick (less than 5ms), and the result isn’t reused often — compute it.

When to Cache

Cache when the data is expensive to compute and rarely changes.

Use cases:

• Product lists
• Blog posts
• Expensive API responses

Example:

const cache = new Map();

function getProductList() {
  if (cache.has('products')) {
    return cache.get('products');
  }

  const products = fetchProductsFromDB(); // slow
  cache.set('products', products);
  return products;
}

Bonus: Use Redis or Cloudflare Workers KV for distributed caching.

How to cache right:

• Set TTL (Time to Live) based on update frequency
• Invalidate smartly — not blindly
• For frontend: use SWR (stale-while-revalidate)

When to Preload

Preload when you know the user will need it soon — but not instantly.

Think of it like laying down tracks for a train before it arrives.

Use cases:

• Preloading the next blog post
• Prefetching dashboard data after login
• Loading images off-screen (lazy loading with hints)

Example:

<link rel="preload" href="/next-page-data.json" as="fetch" />

Or in JS:

const preloadData = () => {
  fetch('/next-page-data.json');
};

Quick Cheat Sheet:

• Compute: Personalized or fast-changing? Compute.
• Cache: Expensive and reusable? Cache.
• Preload: Predictable future use? Preload.

Real Dev Tips

• Use Lighthouse to identify preload opportunities
• For SSR apps, cache API calls server-side with Next.js Incremental Static Regeneration
• Don’t cache HTML unless it’s static content — instead cache API responses

Ask Yourself Before Choosing:

1. Will this data be reused across sessions or users? → Cache.
2. Is it personalized or time-sensitive? → Compute.
3. Can we predict its need before it’s needed? → Preload.

Let’s Talk

Have you made a caching decision that backfired? Or found a smart preload trick that saved seconds?

Share your experiences in the comments — your insight might help someone out there avoid a costly mistake.

Want more tips like this on web performance, architecture, and design thinking?

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This content originally appeared on DEV Community and was authored by DCT Technology Pvt. Ltd.