Interaction to Next Paint - Desktop/Mobile Discrepancy

[A.J. Maciejewski]

Hello.

I use the PageSpeed Insights tool daily and have impeccable scores for our website.

However, there is a huge discrepancy between the mobile and desktop scores for Interaction to Next Paint even though there is virtually no difference in the mobile and desktop versions of our website.

I read the documentation for Interaction to Next Paint and I'm confident that I am doing everything possible to minimize the INP time.

With all of this considered, I'm inclined to believe that there is something inherent about mobile devices that inflates this number. If this is the case then perhaps the threshold for mobile should accommodate for this?

I attached screenshots of my results. Please let me know if I'm overlooking something or if I'm onto something with this.

- A.J.

[Michal Mocny]

Hi A.J., thanks for reaching out with feedback!

• Note that the screenshots show that there was not enough traffic for this URL, so you are seeing Origin data.
• That means that the INP issues could have happened on another page.
• I did try to spend a few minutes, and I was not able to reproduce any significant INP issues (desktop or mobile), though I did not try lower-end devices (good job on your metrics!).

At this point I would be curious to know which set of users actually reported such high INP scores.  Is it a specific geography or device class?  Perhaps your site is particularly popular among a specific set of users?  I believe you could get more insights from the CrUX API and/or by measuring your own field data (e.g. using web-vitals.js).

Someone is reporting such high INP scores, and the first step is understanding when/where before trying to figure out why.

-Michal

[A.J. Maciejewski]

Hello, Michal. Thanks!

Yes, I'm aware that is the Origin data which is what made me concerned.

Specifically, it's odd that there's such a big discrepancy between desktop and mobile because it's basically the same site and nearly every page has the same layout.

Also, I don't have many interactive elements besides simple links to other pages, AdSense ads, and embedded YouTube videos.

If I had to guess, I would say that AdSense is causing this and if it is, it's probably the mobile anchor ad at the bottom that emerges after you scroll a bit.

I'll look into some testing but everything I've tried so far makes it seem like it's all fine.

- A.J.

[Philip Walton]

Hey A.J.

It's hard for me to comment on what exactly could be causing your high INP on mobile, but I do want to point out that the discrepancy you're seeing between mobile and desktop for INP score is *very common* and is something we expected to see when we were designing the metric.

Desktop devices tend to be orders of magnitude more powerful than mobile devices, and so they can process code (both app code and browser code) much faster, which leads to faster response times. Mobile devices tend to be much slower, and so the same page running the same code will usually report longer response times for the same types of interactions.

If you want to achieve good INP scores on mobile, then you may want to reconsider how your mobile site is built, how many ads you serve, etc. Obviously this is a personal choice and there are trade-offs to consider.

Also note that part of the reason we created INP was to help identify these slow third-party tags so we could work with them to improve. Hopefully we'll have more to report on that front in the near future.

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