Enhancement: Improve performance by deprioritizing Stats script

[westonruter]

Impacted plugin

Jetpack

What

Add fetchpriority=low to the Stats script and remove the dns-prefetch link

How

To attempt to reduce network contention with loading resources in the critical rendering path (e.g. the LCP element image), I suggest deprioritizing the Stats script by:

1. Adding fetchpriority="low" to the script tag.
2. Removing the dns-prefetch for stats.wp.com.

This does not primarily benefit Chrome since that browser already gives async scripts a priority of low. It does benefit Safari and Firefox, however, since they have a default medium/normal priority.

I created an extension plugin to demonstrate the changes: Jetpack Stats Script Deprioritization.

I published a full writeup as well: Improve LCP by Deprioritizing Script Modules from the Interactivity API.

[jeherve]

Thanks for the contribution! It definitely seems like a good improvement for those browsers.

I'm not 100% clear on removing dns-prefetching though. Wouldn't that still be beneficial?

[westonruter]

Since the stats script isn't needed in the critical rendering path, it doesn't seem beneficial to do the dns-prefetch since that would just add network contention for other more critical resources. If the DNS lookup takes longer when the script is loaded in the footer, that should be totally fine and even desirable.

[jeherve]

That makes sense.

Since you offered a patch for the Core ticket, do you have any sense of when it may be committed? I'm wondering if we should wait for the new option to be added to wp_enqueue_script, or use wp_script_attributes today like you've done in your extension plugin.

[westonruter]

My hope is that this can be part of the next minor release, given the relaxation in features being part of such releases.

[kraftbj]

I prefer to add this once WordPress/wordpress-develop#8815 drops in Core for the fetchpriority. As a progressive enhancement, we can make the change without worrying about the minimum version.

It appears that stats.wp.com is not part of the prefetch (anymore at least)