Async loading allows your visitors to view app content right away when the site is loaded.

Normally, this is the page loading lifecycle:
1. Page visible with the design you made inside the Editor

2. Run page code (including widgets code)

3. Visitors see the page with modification made from the code, e.g. additional site content, changed color, dynamic content like 'current time display with ticker'.

With async loading, it becomes:
1. Run all page code, and wait for completion for final state.

2. (Step 1 and Step 3 from the normal lifecycle) happens at the same time, so there is no 'waiting' to see modification

Benefits:
User can see the final content and have everything interactive and playing already once page become visible

Issue:
Some app widget can takes a long time to load. Some might rely on third party services and live data (e.g. stock prices, social media content with large file sized videos) which cause your site takes a long time to load. When failed to load, you might see a white screen on your site randomly.

To enable async loading, locate the app settings and enable it.
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Learn more: https://support.wix.com/en/article/viewing-your-pages-source-code-the-way-search-engines-do

To improve loading time, read: https://support.wix.com/en/article/site-performance-caching-pages-to-optimize-loading-speed