[Support request] CLS problem with slideout overlay and page hero

[Norman]

So, I’ve spent the past half hour looking through the previous CLS topics but sadly I was not able to find an exact solution for my problem. But if there is, kindly point me towards it.

I seem to have two major problems with CLS

1) div class=”slideout-overlay”
I tried to change to flexbox but that didn’t really fix the issue. Am I missing something?

2) div class=”page-hero grid-container grid-parent”

The hero also seems to generate an LCP problem on top of the CLS issue.

How do I fix these two issues?

There is also a problem with “div class=”site grid-container container hfeed”” but from what I gathered that’s related to my ads and probably nothing I can (read want) to fix.

[Norman]

Hm..anyone out there?

[David]

Hi there,

your topic slipped past our system – so please accept our apologies for not replying sooner.

Can you disable the Optimized CSS delivery (Critical CSS) option in WP Rocket – the automatic method is one of the main causes of CLS that we see.

[Norman]

Okay..that seems to have done something. However, I still get the LCP issue for div class=”page-hero grid-container grid-parent”

[David]

Is the background image in the Header Element the Featured Image?

[Norman]

yes it is.

[David]

Try adding this PHP Snippet:

add_action( 'wp_head', function() {
    if( is_single() ) {
        if ( wp_is_mobile() ) {
            $featured_img_url = get_the_post_thumbnail_url(get_the_ID(),'medium_large'); 
            echo '<link rel="preload" as="image" href="'.$featured_img_url.'"/>';
        } else {
            $featured_img_url = get_the_post_thumbnail_url(get_the_ID(),'full');
            echo '<link rel="preload" as="image" href="'.$featured_img_url.'"/>';
        }
    }		
});

How to add PHP: https://docs.generatepress.com/article/adding-php/

This will preload the background image, including a smaller attachment size for Mobile.

[Norman]

Hm..tried that but doesn’t seam to work 🙁

[David]

well the code is working as the image request is now being made before the HTML is loaded….
But the file its outputting is still 183kb … how large was the original image in file size ?

[Norman]

The image is 1200 by 801 pixels and the file size is 188kb.

[David]

Well Google PSI is definitely having a turn on this one… LCP on background images are awfully hit and miss.
As this cannot be explained:

If you check the first frame of the filmstrip highlighted in Red box you can see the LCP ( featured image ) element. If there were the actual delay between FCP and LCP time then that featured image would not be visible in the first frame… going by the FCP/LCP timings i wouldn’t expect to see it anywhere near the first frame.

Which makes it really hard ( impossible ) to debug. We’re preloading the featured image, as you can see here in the waterfall report:

https://www.webpagetest.org/result/210511_BiDcGE_ae9694fae6582736726d45afd843af48/1/details/#waterfall_view_step1

its the first request to be made after the HTML has loaded. So theres not a request delay.

So this leaves it down to two possiblities ( in my mind anyways ):

1. The size of the image – google may just be ‘simulating’ that image takes longer to display then it is.
Reducing the size of the image would help.

We could try a quick test, in the code above you will see:

medium_large and large – change them both to medium so we can see if it pulls in the smallest of images.

2. Inteference from other scripts – aside of the unexplainable film strip – i notice every PSI report i ran there were completely different opportunities being flagged. One of the common one was the ezoic ad scripts causing render blocking. You can try deferring those scripts using a plugin like Flying Scripts.

In addition to this – running multiple tests i get various results and various opportunities. They never seem to be the same.

[Norman]

Okay..you seem to be switching up some things here.
– I am not with Ezoic, so I doubt there is Ezoic code running 😉
– In your PHP code there was no large, just a full. I changed that to medium as well. Tho no change at all.

[David]

Aah my bad – false of habit – i meant mediavine ( just saw an Advert script ) appearing in a render blocking opportunity in PSI.

Remove the above snippet and try adding this instead:

add_action( 'wp_head', function() {
    if( is_single() ) {
        $featured_img_url = get_the_post_thumbnail_url(get_the_ID(),'medium'); 
        echo '<link rel="preload" as="image" href="'.$featured_img_url.'"/>';
        echo '<style>
        @media(max-width: 500px) {
            .page-hero { background-image: url('.$featured_img_url .');}
        }
        </style>';
    }		
});

[Norman]

doesn’t seem to work either. It does seem to improve the speed score on two out of three requests, tho. God…I hate these core vitals

[David]

Well that code is definitely an improvement. On mobile networking test – the image is preloaded ( first in the request queue ) and its a tiny 13kb. Thats a 170kb saving! It loads so fast it actually appears before the logo….
Tested on Lighthouse locally the FCP is 1.3s ( matching Google PSI ) and the LCP is a consistent 2.2s.

But we still see the anomaly on PSI.

So the only thing i can consider is that ‘something’ else is interfering with the API that measures the LCP element. I still see the Mediavine CSS being preloaded ( the browser complains its preloaded but not used with a short time ) and then theres all the scripts that they load.

A simple to test to determine whether the advert code is interferring would be to temporarily disable it.

As other than this – i am at total loss !

[Author]

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