FID warnings while getting "100" in PageSpeed

[guarriman3]

Hi,

I manage a website with two sections:
- mysite/product/name (80% of the URLs)
- mysite/category/name (20% of the URLs)

Within my Google Search Console's 'Core Web Vitals' section, the URLs of the 'product' section are good/green, while the 20% of URLs of the 'category' section are yellow (need improvement). The issue seems to be "FID issue: longer than 100ms (mobile)".

Apparently, both 'product' and 'category' sections are built with the same architecture and HTML elements. However, the average FID of the URLs of the 'category' section is 125ms.

I started to analyze the issue to find out the origin of the problem:
- tested with 'PageSpeed Insights', and got always "100" for any URL of the 'category' section, with values of "Total Blocking Time" ranging from 0 to 20ms.
- tested with Chrome's Lighthouse, with same results
- tested with 'web.dev' tool, with same results

I'm not sure how to research the origin of this FID issue. Any tip is appreciated. Thank you.

[robzilla]

You'll often find differences between lab data and field data. It's possible your real users are on relatively slow devices, for example.

I wouldn't worry about it from a rankings perspective, but if you want to improve performance, you can use the Performance tab in Chrome to dig deep into the loading and rendering process. If you can't find anything of significance, I would just ignore it. Yellow is fine, red could be a problem, although as discussed recently the effects of the page experience update are likely to be minor.

[guarriman3]

Hi @robzilla, thank you very much for your nice answer. Your words put my mind at ease.

I have been trying for many months to get all the pages in green. I analyze the "Peformance" tab in Google Chrome, and try to split JavaScript tasks by using lots of "setTimeout". Sometimes (for the same page, it occurs occasionally and infrequently), I see a red flag (100-200ms) for tasks related to 'www.googletagservices.com' (I run AdSense ads), but I guess they are beyond my reach.

[robzilla]

Yes, AdSense will slow you down; there's a trade-off there. By how much depends on the ads that are loaded (the number as well as the format, e.g. text-based ads vs. video ads), and since these are highly personalized that's probably another reason you'll find differences between the lab and field data. If you can't find any significant opportunities for performance improvement in your own code, you can safely assume AdSense is the culprit. And you're right, that's mostly beyond your reach.

[guarriman3]

Does it make sense if I include DNS-Prefetch of the domains related to Adsense to reduce some time those Javascript requests?

[robzilla]

It might help to prefetch DNS for and/or preconnect to domains that are always employed, but I wouldn't expect significant gains. Most of the time is lost in loading and executing script chains, and loading the contents of the ads; DNS and connect times are peanuts compared to that.