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WPT Speed Index versus LH Speed Index
11-06-2019, 07:54 PM
Post: #1
WPT Speed Index versus LH Speed Index
We notice that there is always a substantial difference between the WPT and LH speed index scores.

If I look at the Google LH documentation for Speed Index it refers to the WPT documentation suggesting that they have the same implementation.

If the technical implementations are slightly different and the scores were within a 5% interval I would think this is okay, but our testing is showing a huge difference. Normally at least 30% difference (WPT is almost always a lower score), but sometimes there is a difference of over 70% for the same test.

Is there a good explanation of the reason for the differences?
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11-07-2019, 12:14 AM
Post: #2
RE: WPT Speed Index versus LH Speed Index
Most likely cause is that Lighthouse always runs the test at "3G Fast" speed because that's what the Lighthouse scores are calibrated for. If you run the WebPageTest tests at different speeds you will get different results.

The other possibility (if both are tested at the same speed) is that the content on the screen shifts towards the end of the test (like a banner ad moving content down). The Lighthouse version of Speed Index is extremely sensitive to content moving while the WebPageTest version is not.
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