I think there actually is a problem, which causes Google to rate your pages sometimes worse then what they actually are. BTW: thanks for using static sites and building pages that scales and perform good… thanks!
Here is what I think causes the problem:
- your “Image Sprites” are loaded AFTER the CSS is loaded and parsed.
- while this itself is not a Problem, the Image Sprites itself seems to be the problem.
Looking at your Google Report it seems unreal how you site does even take longer then 1s to load. But yet Google is reporting LCP (Largest Contentful Paint) of more then 2.5s which is strange.
But after looking how your Image Sprite loads globaly it makes more sense:
In some regions it triggers a 403 Error and even does have loading timings of (this one image itself) of more then 2.5s (NY = 6.4s). Seems like somthing is not correct with this image or how it gets served. I personally would look into this first to find out why Google marks your pages as having a high FCP.
NOTE: this test was done after some hard-reloads and it also depends on from where Google crawls your page. I personally would be happy about a more detailed log/info from Google, but thats how it is.