Why does dynamically assigning src to iframes through JavaScript lead to slowing down rendering?

Analysis of rendering differences between dynamic assignment of iframe src and static assignment of static assignment

In web development, iframes are often used to embed external web pages. However, dynamically modifying the src attribute of an iframe using JavaScript, the rendering speed difference is obvious compared to static assignment in HTML. This article will analyze its causes and solutions.

Problem: Dynamic assignment of src causes rendering to slow down

The developer observed that when dynamically modifying the iframe src attribute, the iframe content renders slowly, showing a top-down progressive loading rather than a whole simultaneous display, which is in contrast to the situation of static assignment.

Cause analysis: Differences in browser rendering mechanism

The browser rendering process is top-down. When statically assigning src , the browser parses the iframe tag and starts loading and rendering its content immediately, synchronizing with the rendering of other page elements, so the user experience is smooth.

When dynamically assigning src , the iframe has completed the initial rendering, and subsequent content loading and rendering are carried out independently, causing the user to perceive the progressive loading of the iframe content, which makes it feel slower. This is not really a slowdown, but a difference in rendering timing and user perception.

Solution: Loading indicators to improve user experience

To improve the user experience, it is recommended to display a loading indicator during the iframe loading process. After the iframe content is fully loaded, hide the indicator. This can effectively alleviate users' negative perception of loading speed.

Sample code:

 iframeLoad() {
  this.loading = true;
  const iframe = this.$refs.iframe;
  // Compatibility processing if (iframe.attachEvent) { // IE browser iframe.attachEvent("onload", () => {
      this.loading = false;
    });
  } else { // Other browsers iframe.onload = () => {
      this.loading = false;
    };
  }
}

Through this method, users can obtain clear visual feedback and improve the overall user experience.

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