At a minimum, what Speed index score should you be aiming for?
Greater than 5000
Within 10% of similar sites
Greater than 95%
Less than 5000
Useable Time: Customers want to see visible content that is “above-the-fold” as soon as possible. Load important content first and make sure that the site is usable in 3 seconds. Webpage test generates the speed index – the time to visual complete above the fold. Our goal is to ensure that the score is below 3000.
Read more here: https://support.google.com/partners/answer/7336284