SpeedIndex not accurate with non-standard Lantern settings

[patrickhulce]

Provide the steps to reproduce

1. Run LH on https://facebook.com or any URL really

What is the current behavior?

Speed Index is overinflated by ~70%

What is the expected behavior?

Speed Index is more inline with FCP/FMP

Environment Information

• Affected Channels: Primarily LR desktop, CLI
• Lighthouse version: *
• Node.js version: *
• Operating System: *

Related
#6836 #6969

[patrickhulce]

The reason here is that our Lantern coefficients are static and it's the only metric where we use the raw observed value as the input to the final simulated metric value. We need to be scaling the real coefficient based on how similar our observed run was