I am testing the WebSite X5 Blog component (version 2025.3.9) and analyzing Core Web Vitals using Lighthouse (mobile simulation).

I have identified a consistent performance pattern affecting blog article pages.

1. Test environment

• WebSite X5 version: 2025.3.9
• Blog component: default configuration (no custom scripts)
• Multiple templates tested
• Clean projects + minimal test articles used
• No external optimization plugins or custom JS

2. Observed behavior

Across all blog article pages, the following pattern is consistent:

Performance metrics:

• Time to First Byte (TTFB): ~0 ms
• Resource load delay: 0–190 ms
• Resource load duration: minimal
• Total Blocking Time (TBT): ~0–20 ms
• Cumulative Layout Shift (CLS): 0
• First Contentful Paint (FCP): ~1.8–2.9 s
• Largest Contentful Paint (LCP): ~2.9–4.3 s

3. Critical issue: Element Render Delay

Lighthouse consistently reports that the main cause of LCP degradation is:

Element Render Delay (~1.2s to ~2.3s)

This delay occurs even though:

• the LCP element is already present in the DOM
• there are no long main-thread tasks (TBT is negligible)
• network loading is not a bottleneck

4. LCP element variation

The LCP element alternates between:

• article text inside .imBlogPostBody
• blog cover image .imBlogPostCover

In both cases, the same render delay behavior is observed.

5. Additional observation

The blog cover image is rendered with dynamically computed inline styles (for example, inline height values such as style="height: 107.478px"), suggesting runtime layout calculations.

This indicates that the layout may not be fully resolved at initial render time.

6. Reproducibility

This behavior is consistent across:

• different blog templates
• fresh projects
• minimal test articles (plain text only)
• fixed and variable image height modes
• different breakpoint configurations

7. Hypothesis (for clarification)

It appears that blog article rendering may involve a post-DOM layout stabilization phase before the first paint of the main content.

This could involve:

• breakpoint evaluation
• dynamic image scaling calculations
• layout alignment processing
• JavaScript-driven container adjustments

8. Question to developers / support

Could you please clarify the following:

1. Is the observed Element Render Delay in blog articles expected behavior in the current Blog engine?
2. Does the Blog system perform a layout stabilization phase before first paint, which could affect LCP metrics?
3. Is there any recommended configuration or optimization strategy to reduce this render delay and achieve more “static-like” rendering behavior for blog content?
4. Are there known architectural constraints in the Blog component that impact Core Web Vitals (specifically LCP render delay), even when network and CPU performance are not limiting factors?

Objective

The goal is to understand whether this behavior is:

• a configuration-level optimization issue
  or
• a structural characteristic of the Blog rendering engine

Any technical guidance on improving blog article LCP performance would be highly appreciated. 

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