Bad score on PSI and Web page test. Slow FCP and LCP

[bitforcesrl]

As you can see in the film strip of web page test the rendering is happening very late.
There is about 1 sec of time spent downloading the blocking CSS and then there are other 600ms after the css has been downloaded to start the page rendering (it seems too much to create the CSSOM and apply CSS and layout/paint/composite). This leads poor performances

• We could decrease the shopify dns resolution and connection returning the CSS in line in the page (baybe this could cause problems with the SPA navigation)
• We've added the preconnect for cdn.shopify but this hints seems not applied by the browser
• The source location of the test is form US but that should not matter since oxygen and storefront api runs on the edge
• we've done some test preloading the image for the LCP but it doen't affect the time of the FP(first paint) it is seems delayed for some reason

Any suggestion to improve this issue? thanks