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According to core web vitals there are only 3 core vitals for measuring the user experience of any website LCP(Largest contentful paint), FID(First input delay) and CLS(Cumulative Layout shift). According to Pagespeedinsights or CRUX dashboard, FID of my website is in good limits i.e 90% of users have an input delay of less than 100 ms

Will there be any benefit if I do the chunk optimisations(splitting, lazy loading) on the user experience of people landing on my website?

I understand that it will effect TBT(Total Blocking Time), TTI(Time to interactive) but anyways it doesn't matter if my FID is ver less. is my understanding correct?

Aman Jagga
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I work on several large sites and we measure FID and TBT across thousands of pages. My work on this shows there is little correlation between TBT and FID. I have lots of pages reporting TBT of 2s or more but then are in the 90% score for FID. So I would NOT spend money or time optimizing TBT, what I would do instead is optimize for something that you can correlate to a business metric. For instance, add some user timings to measure how fast a CTA button appears and when it becomes interactive. This is a metric that is useful.

Tim Bednar
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Being in the green on the core web vitals report (for one or all metrics) is great, but it doesn't mean that you should not try to improve performance further. In fact, if all your competitors have better FID / CLS / LCP / etc. than you, you will be at a disadvantage. Generally speaking, I think the web vitals report can be used as a guide to continuously prioritise changes to try and improve performance.

While it's impossible to predict the improvements without looking at a current report and the codebase, it seems fair to expect code-splitting to improve FID and LCP, and lazy-loading to help with LCP. Both improvements would benefit users.

Note that TBT and FID are pretty similar.

Jelefra
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