為何網站載入緩慢：開發者應修正的 7 個效能問題

A website can look beautiful, contain useful content, and offer excellent features, yet still frustrate users if it takes too long to load.

Website performance is not just a technical concern. It affects how people interact with your application, whether they complete important actions, and how reliable your product feels.

The good news is that many performance problems come from a handful of common development mistakes.

Let's explore seven of them and see how developers can address each one.

1. Using Unoptimized Images

Images often account for a significant portion of a webpage's downloaded resources.

Uploading large images directly from a camera or design tool can unnecessarily increase page size.

How to fix it:

• Use modern formats such as WebP or AVIF when supported by your workflow.
• Resize images to match their actual display dimensions.
• Compress images before uploading them.
• Use lazy loading for images that appear below the fold.

For example:

<img
  src="/images/product.webp"
  alt="Product preview"
  width="640"
  height="480"
  loading="lazy"
/>

Use lazy loading for non-critical images. Avoid applying it to the main above-the-fold image when that could delay the content users see first.

2. Loading Too Much JavaScript

JavaScript enables interactive interfaces, dynamic content, and complex application features. However, shipping unnecessary JavaScript can increase download, parsing, compilation, and execution time.

This is particularly noticeable on slower devices.

How to fix it:

• Remove unused dependencies.
• Split large bundles into smaller chunks.
• Load non-essential features only when needed.
• Use code splitting and lazy loading where appropriate.
• Check whether a simple HTML or CSS solution could replace unnecessary JavaScript.

In a React application, a component that is not needed immediately can be loaded on demand:

import { lazy, Suspense } from "react";

const Dashboard = lazy(
  () => import("./Dashboard.jsx")
);

function App() {
  return (
    <Suspense fallback={<p>Loading dashboard...</p>}>
      <Dashboard />
    </Suspense>
  );
}

export default App;

This can reduce the JavaScript required for the initial page, although the actual benefit depends on the application's structure and bundler configuration.

3. Ignoring Browser Caching

Without effective caching, returning visitors may repeatedly download resources that have not changed.

Caching can reduce unnecessary network requests and improve repeat visits.

How to fix it:

• Configure appropriate Cache-Control headers.
• Use long-lived caching for versioned static assets.
• Revalidate resources that may change frequently.
• Avoid caching personalized or sensitive responses publicly.

For example, a versioned JavaScript file could use a header such as:

Cache-Control: public, max-age=31536000, immutable

This is suitable only when the asset URL changes whenever its content changes. HTML pages and user-specific responses generally require different caching strategies.

4. Making Too Many API Requests

A page may feel slow even when its frontend code is efficient if it waits for several unnecessary API calls.

For example, a dashboard might request user details, notifications, statistics, and profile information separately—even when some requests could run concurrently or be combined safely.

How to fix it:

• Identify requests that do not depend on one another.
• Run independent requests concurrently.
• Avoid repeatedly requesting the same data.
• Consider pagination for large datasets.
• Use appropriate caching and request deduplication.

Example:

async function loadDashboard() {
  const [profile, stats] = await Promise.all([
    fetch("/api/profile"),
    fetch("/api/stats")
  ]);

  if (!profile.ok || !stats.ok) {
    throw new Error("Unable to load dashboard data");
  }

  return {
    profile: await profile.json(),
    stats: await stats.json()
  };
}

Concurrent requests can reduce waiting time when the requests are independent. Always handle errors and avoid exposing sensitive information through client-side requests.

5. Running Inefficient Database Queries

Backend performance can become a bottleneck when database queries retrieve more data than necessary or repeatedly scan large datasets.

Common causes include missing indexes, unnecessary columns, inefficient joins, and the N+1 query problem.

How to fix it:

• Retrieve only the columns you need.
• Add indexes based on actual query patterns.
• Inspect slow queries with database profiling tools.
• Use pagination instead of loading entire tables.
• Check whether repeated queries can be consolidated.

For example, instead of retrieving every column:

SELECT *
FROM products
WHERE category_id = 12;

Select only the required fields:

SELECT id, name, price
FROM products
WHERE category_id = 12;

The second query reduces unnecessary data transfer when the application needs only those three fields. Whether it improves execution speed also depends on the database schema, indexes, and query plan.

6. Blocking the Browser With CSS and JavaScript

Some resources can delay the browser from rendering the page.

Large stylesheets, synchronous scripts, and poorly organized frontend assets can contribute to a slower initial experience.

How to fix it:

• Remove unused CSS.
• Minify and compress production assets.
• Use defer for scripts that do not need to execute before HTML parsing finishes.
• Load non-critical resources strategically.
• Avoid excessive third-party scripts.

For example:

<script src="/js/app.js" defer></script>

The defer attribute allows the browser to continue parsing HTML while downloading the external script. Deferred scripts execute after parsing finishes and before the DOMContentLoaded event.

Always test script dependencies before changing loading behavior.

7. Never Measuring Performance

One of the biggest mistakes is making optimization decisions based entirely on assumptions.

A page might feel slow because of a large image, an expensive JavaScript operation, a slow server response, or a third-party script. Each problem requires a different solution.

How to fix it:

Use browser developer tools and performance measurement services to investigate the actual bottleneck.

Useful tools include:

• Chrome DevTools: Inspect network requests, JavaScript execution, and rendering.
• Google PageSpeed Insights: Evaluate performance and user-experience metrics.
• Lighthouse: Audit performance and other quality areas.
• WebPageTest: Investigate loading behavior under different test conditions.

Pay particular attention to Core Web Vitals:

• Largest Contentful Paint (LCP): Measures when the main content becomes visible. A good result is 2.5 seconds or less.
• Interaction to Next Paint (INP): Measures responsiveness to user interactions. A good result is 200 milliseconds or less.
• Cumulative Layout Shift (CLS): Measures unexpected visual movement. A good result is 0.1 or less.

These thresholds apply to the 75th percentile of page visits, segmented by mobile and desktop where applicable—not to every individual visit.

When Should You Get Help With Website Optimization?

Some performance problems are easy to fix, such as compressing images or removing an unused dependency. Others require deeper investigation into the frontend architecture, backend services, database queries, hosting environment, and application traffic.

If your team has identified performance issues but lacks the time or expertise to address them, working with an experienced development team may help.

For businesses planning a new website or improving an existing application, Oneslogic's web development services can be explored as one option for building and optimizing websites with performance, usability, and scalability in mind.

The right approach depends on your current bottlenecks, technical requirements, and budget. Start with evidence rather than assuming that a complete rebuild is necessary.

Final Thoughts

A slow website rarely has just one possible cause. Images, JavaScript, caching, API requests, database queries, and browser rendering can all contribute to a poor experience.

The most effective strategy is to measure first, identify the biggest bottleneck, make one targeted improvement, and measure again.

You do not always need a new framework or a complete redesign to make a website faster. Sometimes, a few carefully chosen changes can make a meaningful difference.

What is the most common performance problem you have encountered in a real project? Share your experience in the comments.

Editorial note: This draft was prepared with AI assistance. Review and fact-check it before publication, and follow DEV.to's current AI-assisted content disclosure guidelines.