why do lcp values differ dramatically between the lighthouse tab and the performance tab for the same test run?

When you view the original trace that is “as it happened” during your audit.

Now if you have a particularly good connection with low latency you will get slight differences.

This is because lighthouse (in the browser at least) applies network throttling, even on desktop.

It sets latency to 40ms and throughput to 10,240 Kilobits per second (so about 1.25 megabytes per second).

This is why you see different numbers, the page you linked is 1.8 Megabytes so will take about 1.5 seconds to download under those simulated network conditions. I am guessing your connection is many times faster than 10 Megabits per second and your latency is probably lower than 40ms if you have a decent connection.

How to get the numbers to match?

On the Lighthouse tab in developer tools you can switch this throttling off.

Just uncheck “Simulated throttling” and rerun the test, you will find your results match exactly as no network throttling is applied.

simulated throttling checkbox located next to clear storage checkbox

If you uncheck “Simulated throttling” on mobile you will get throttling that is applied so you will actually see the page load slower. Yet again your trace will match exactly.

You can see all of the throttling information at the very bottom of your report under “Runtime Settings”.

Runtime Settings example

As you can see I have 0 ms HTTP RTT, 0 Kbps down, 0 Kbps up (DevTools) for network throttling as no throttling is applied. You probably see the 40 ms TCP RTT, 10,240 Kbps throughput (Simulated) that I mentioned earlier.

Page Speed Insights

Whether they use the exact same numbers on Page Speed Insights I am unsure but they likely apply some throttling as a multi-gigabit dedicated connection like they have is not indicative of a real world home broadband connection.

They may also have slight CPU throttling as a server processor would not be indicative of a laptop / PC processor.

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