I've got 2 almost identical pages: one for brand, and one for category.
Brand looks exactly like category, and has same number of products.
Brand page gets score 64/95, and category gets score 88/80.
There is Amasty FPC installed, and both pages are served from cache, page load time is the same: 0.4 sec.
The only difference I see at pagespeed is this:
Brand mobile: PSI estimates this page requires 7 render-blocking round trips and 54 resources (1.2MB) to load.
Brand desktop: PSI estimates this page requires 1 render-blocking round trips and 58 resources (1.1MB) to load.
Category mobile: PSI estimates this page requires 1 render-blocking round trips and 59 resources (1.2MB) to load.
Category desktop: PSI estimates this page requires 1 render-blocking round trips and 63 resources (1.4MB) to load.
Note that at mobile brand it shows 7 round trips, and 1 everywhere else. I assume this is this issue. But mobile brand and desktop brand are the same page.
All pages have the same 3 minified CSS, and 1 minified JS.
There are both links:
Note that if you check the scores, load them twice, to prime the cache.