Increase default for `wp_omit_loading_attr_threshold` to 3

[flixos90]

The current default threshold for how many content images to skip lazy-loading on (which is just 1) has proven to be too strict: Many sites have more than 1 large image and potential LCP candidate visible above the fold, and due to the very conservative threshold here, those sites may end up lazy-loading their LCP image which negatively affects load time performance.

Based on HTTP Archive data queried to get a better idea of this (see attached screenshot), a reasonable default value for the filter would be 3. >70% of sites have up to 3 equal-sized images in the initial viewport, each of which could be the LCP image and therefore should not be lazy-loaded. Sites that have fewer than 3 images above the fold will not see a negative performance impact, they will only lose out on 1-2 images that may otherwise have been lazy-loaded to be loaded immediately, which is an acceptable tradeoff to get faster overall load time performance.

While we could of course increase the default to even a higher number, the % of sites that use a specific number greater than 3 is much lower, e.g. to cover 80% of sites in total, the default threshold would need to be set to 8, which is excessive and would reduce the lazy-loading benefits too much for the majority of sites.

[flixos90]

HTTP Archive data screenshot for how many sites have a given number of LCP candidates

[flixos90]

Addressing the remaining lazy-loading related performance issues is a high priority for the performance team for the 6.3 cycle.

Trac ticket: https://core.trac.wordpress.org/ticket/58213

Increase wp_omit_loading_attr_threshold threshold to 3

@felixarntz This is good for review. I have fixed both the test error (​here) by using wp_omit_loading_attr_threshold filter.

[spacedmonkey]

Marking as ready to commit and assigning to @flixos90 to commit.

[flixos90]

In 55816:

Media: Increase default for wp_omit_loading_attr_threshold to 3.

The previous default threshold for how many content images to skip lazy-loading on (which was just 1) has proven to be too strict: HTTP Archive data shows that >70% of sites have up to 3 equal-sized images in the initial viewport, each of which could be the LCP image and therefore should not be lazy-loaded. Lazy-loading too many images has adverse effects on load time performance, while increasing the default threshold will not negatively affect load time performance even for sites where a threshold of 1 would be the perfect choice.

The change of default value in this changeset will improve performance for more WordPress sites out of the box. The wp_omit_loading_attr_threshold filter can still be used to customize and fine tune the value where needed.

Props thekt12, spacedmonkey, westonruter, flixos90.
Fixes #58213.

Committed in https://core.trac.wordpress.org/changeset/55816 🎉

[flixos90]

Dev note published: ​https://make.wordpress.org/core/2023/07/13/image-performance-enhancements-in-wordpress-6-3/