PageSpeed Insights - Sometimes huge Total Blocking Time (TBT)

Hi all,

we just started to optimize our site for performance and Google Core Web Vitals. Still, we need to do a lot of code optimization, but one thing I don’t understand.

A very simple and basic test page shows sometimes 100 performance scores and sometimes down to 60. See attached screenshots.

The site URL:

Chrome Developer Tool LightHouse constantly reports low TBT.

Perhaps off-topic: PageSpeed Insights reports some Cumulative Layout Shift (CLS). LightHouese does not.

Any explanation?


Hey @don-esteban,

TBT (or CLS) issue is not something that can be caused by Netlify - it has more to do with the client-side working of your application than server side. You need to profile your app using the Performance tab and see how you can optimise it further.

Hi @hrishikesh,

thanks for Your advice. I misinterpreted TBT. But still, how can it happen? Subsequent PageSpeed responses show that huge differences? It can’t be the App. Right? If it is not the CDN. Is it PageSpeed Insights? I just don’t have any clue where to start optimization. Sometimes, I get TBT down to 0ms and sometimes up to 6 seconds.

Those values by TBT is basically time taken by the client browser - this can vary from device to device. On a very high performing device, it could be very low, on an overloaded, system it can shoot up. You need to test your website on multiple devices and see how it performs in real-life instead of relying on values given by page-speed insights. Sure, those values help, but if you’re at a point where you feel you can’t optimise your code any further, you should really ask yourself if getting a 100 in PSI is worth it.

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Thanks, @hrishikesh. Sure, I’ll do. The 100 points are not a goal. They are an implication of a test page basically doing nothing. :slight_smile: But showing the expected 100 points sometimes and seconds later showing only 50 points made me suspicious. BTW. LightHouse Chrome Dev Tools reports constantly good values.

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