WPT Speed Index versus LH Speed Index

[pmeenan]

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.