TL;DR: JavaScript-heavy websites often underperform or fail to load entirely for users in mainland China due to factors including blocked third-party scripts, high network latency, and incompatible CDNs. Without optimization, these issues degrade site speed, functionality, and overall user experience. Chinafy helps remediate these challenges by optimizing both code and infrastructure for web performance in China.

How do I know if JavaScript is slowing down my website in China?

To test your website in China, you can use tools like Chinafy’s visual speed test, global speed test, or WebPageTest.org.

Then you can check the waterfall chart to identify JavaScript files with long load or execution times.

Look out for:

Long load or execution times for .js files

High Time to First Byte (TTFB)

Heavy CPU usage, indicating processing delays

Does JavaScript work in China?

Some JavaScript files fail consistently in China, such as Google Maps API files. Others load inconsistently, like Google Tag Manager. And some load but do so slowly or unreliably.

Here’s an example of a JavaScript file that loads in China, but impacts loading time and interactivity on the page, particularly when there are multiple files like this:

Source: Webpagetest.org

The file above has a load time of 995 ms and TTFB (time to first byte) of 711 ms.

Its significant CPU time (130 ms) means the browser spent 130 milliseconds processing the resource, significantly more than the main document’s 44 ms, indicating this resource requires substantial parsing or execution (like most JavaScript files). This will likely impact interactivity for the end user.

Although this file on its own is not blocked or broken, the cumulative impact of slow-to-process JS files leads to slower overall website load times and detriment to user experience.

When testing this site in Beijing, China, the load event (marking the completion of all resource loading) fires at 7 seconds. This compares to 4-5 seconds in Canada and the UK.

Common performance issues for JS-heavy sites in China

JavaScript-heavy websites often suffer poor performance in China due to a mix of infrastructural and compatibility challenges.

Blocked scripts can cause timeouts

JavaScript files hosted on inaccessible platforms (e.g., Google APIs, Facebook) delay rendering. Browsers wait for these files to time out before proceeding, creating bottlenecks in page load.

Read more about APIs in China

High network latency and packet loss

All international traffic enters China through a limited number of gateways, causing:

High latency

Intermittent packet loss

Throttling, especially during peak hours

This significantly affects JavaScript files that require multiple external requests.

Script execution delays

Single-page applications (SPAs) with large JS bundles can overload devices with slower CPUs or limited memory. Parsing and execution delays extend the Time to Interactive, especially when uncompressed or poorly optimized.

Overall degraded UX

Combined, these factors can result in sites that load in 10–30+ seconds or fail entirely. Blank content sections, broken functionality, and poor interactivity are common. Studies show that 90% of websites take too long to load in China, well above user expectations of 2 seconds or less.

What causes performance issues for JavaScript in China?

JavaScript files on any website – both inside and outside of China – can take longer to load than other file types (like HTML, CSS, or images) because of factors including:

File size and complexity: JavaScript files often contain complex logic, libraries, or frameworks (e.g., React, Angular), which can make them larger than simpler HTML or CSS files. Larger files take longer to download.

Parsing and execution: Browsers must download, parse, and execute JavaScript files. Unlike static assets (e.g., images or CSS), JavaScript requires the browser to interpret and run the code, which adds processing time, especially for complex scripts.

Network requests: JavaScript files often depend on external libraries or modules hosted on CDNs or other servers, leading to additional network requests. Each request introduces latency, especially if the server is slow or distant.

Render-blocking behavior: Many JavaScript files are render-blocking, meaning the browser pauses rendering the page until the script is fully loaded and executed (unless async or defer attributes are used). This can make the page feel slower compared to non-blocking assets like images.

Minification and compression: Even though minified and compressed JavaScript files can reduce download time, unoptimized or poorly compressed files can significantly increase load times.

Client-side processing: Some JavaScript files trigger additional client-side computations (e.g., DOM manipulation, API calls, or dynamic content generation), which delay the perceived load time.

Caching: If JavaScript files aren’t properly cached (e.g., via browser cache or service workers), they must be reloaded on each visit, increasing load time compared to cached assets like images.

The China context adds another layer of unique factors that can lead to JavaScript having an even greater impact:

China’s unique internet environment: China's internet system inspects and filters international traffic. JavaScript files hosted on servers outside China (e.g., CDNs like Google, Cloudflare, or AWS) may face delays due to deep packet inspection, throttling, or temporary blocks.

Cross-border latency: International websites often host JavaScript files on servers outside China. The limited number of internet gateways (e.g., through Shanghai, Beijing, or Shenzhen) cause high latency for cross-border data transfer, slowing down JavaScript file downloads compared to local assets.

CDN availability: Many global CDNs are unreliable or slow in China as they do not have PoPs (points of presence) directly inside mainland China. If a website relies on a foreign CDN for JavaScript libraries (e.g., jQuery, React), load times increase unless a China-specific CDN (e.g., Alibaba Cloud, Tencent Cloud) is used. These local CDNs require compliance with Chinese regulations, which many global sites don’t implement.

Network congestion and throttling: China's internet infrastructure prioritizes domestic traffic. International connections are often throttled, especially during peak usage times or politically sensitive periods, further slowing JavaScript file delivery.

Blocked third-party dependencies: JavaScript files often depend on third-party APIs or resources (e.g., Google Analytics, Firebase). If these are blocked in China, the script may hang or fail, increasing perceived load times.

How to optimize JavaScript-heavy sites for China

To ensure your JavaScript-heavy website performs well for users in mainland China, you can adopt a combination of technology replacements, hosting strategies, and architectural changes.

Note: it may not make sense to alter your global site specifically for China if it’s a detriment to global visitors.

Or, use Chinafy to tackle JavaScript resources as well as other third-party services and infrastructural incompatibilities leading to poor web performance for your site in China.

There are two main approaches:

1. Manual optimization

This requires significant time and expertise. Recommendations include:

Use China-accessible CDNs or self-host JS

Avoid libraries hosted on blocked CDNs (e.g., ajax.googleapis.com).

Host JS on your own domain or use China-optimized CDNs (e.g., Alibaba Cloud, Tencent Cloud).

Note: China-based CDNs require an ICP license and other prerequisites.

Read more on how to choose a CDN for China

Remove or replace blocked integrations where possible

If you’re tailoring a site to a China audience only, you can audit your site for any integrations that pull from the big blocked domains (Google, Facebook, Twitter, etc.), and remove them or swap them with local equivalents. For instance, you could remove non-critical scripts that hang or block page load.

Minimize and streamline JavaScript

These techniques benefit web performance both in and outside of China. We highly recommend contacting SpeedCurve if you wish to have help with this.

Minify, compress, and bundle files

Use async or defer to prevent render-blocking

Limit large polyfills and unnecessary third-party scripts

Implement caching strategies to reduce repeat load times

2. Optimize JavaScript for China with Chinafy

Rather than rebuild your tech stack for China, Chinafy enables your existing global website to load quickly and reliably for visitors in China, JavaScript included.

How to optimize JavaScript for China with Chinafy

If you’d prefer to maintain your current global website while making your site work in China, including JavaScript files, Chinafy offers a bolt-on, low-touch solution.

Here’s how Chinafy works:

Generates a version of your website for China visitors without affecting your current site and its JavaScript assets.

Identifies and remediates blocked or slow resources, such as handling JS files hosted on Google libraries by replacing, removing or optimizing them.

Adds a China-friendly CDN without changing your current hosting set-up.

Chinafy routes China-IP-based traffic to the China-optimized version of your site without impacting global traffic which will still see your existing site and JavaScript dependencies.

Chinafy then keeps your China-optimized site in sync with your global one, so that any changes to content are reflected accordingly.

Looking to make your site work faster and more reliably for China visitors? Get in touch with Chinafy today for a complimentary website analysis.