Low Pagespeed score for a fast page

[SeriousJ]

Our fast page gets a low "Optimisation score" of 73/100 from Google Pagespeed insights even though there is only one "Optimisation suggestion", while a slowest competitor page with plenty of "Optimisation suggestions" gets a higher score of 81/100.

The only difference is a slightly slower "First paint" time for the fastest page.

Does Google cares almost exclusively about "first paint" time? The rest seems to not matter.

Below is data for the two pages from a far-away server:

Fast page

Google Optimisation score : 73/100

4 render-blocking round trips and 28 resources (0,2 Mo) to load

Optimization Suggestions :

5 blocking CSS resources

GTmetrix data:

Fully Loaded Time: 2.9s

Total Page Size: 312KB

Requests: 35

First paint: 2.4 s

DOM loaded: 2.1 s

Onload: 2.9 s

Slow page

Google Optimisation score: 81/100

5 render-blocking round trips and 126 resources (2.1MB) to load

Optimization Suggestions :

19 blocking script resources and 4 blocking CSS resources

Leverage browser caching for 59 ressources

Minify JavaScript for 1 file

Optimize images for 1 file

GTmetrix data:

Fully Loaded Time: 10.0s 

Total Page Size: 2.27MB 

Requests: 145

First paint: 2.0 s

DOM loaded: 2.9 s

Onload: 6.5 s

[SeriousJ]

Actually we noticed that our Time To First Paint on mobile was faster than our competitor's:

Our TTFP on mobile: 2.7s

Our competitor's TTFP on mobile: 4.6s

So we have a much speedier website than our competitor's on all metrics. And yet, our competitor's Pagespeed score is higher. What's going on?