Welcome, there are several sites on magento2
they are all located on the same server, nginx, php-fpm, ssl, used one copy of magento2 , for each domain - your website, store and starview. 3 of them are on default theme - luma, 2 - on the popular themeforest theme on the market.
In General, the same settings have a wild difference in performance, exactly the problem in the boot, more precisely, as shown by webpagetest, TTFB - time to first bite.
For clarity - the first site on the market is slow, the second to luma and fast.
Website 1
Site 2
As can be seen, first site time to load the first byte is a shame. The logical conclusion that it is in the subject, but a huge number of shops are using commercial themes, including market. I understand if it was unsolvable, no one would use such things, right?
Question - how to solve the problem of how to determine exactly what is happening these 5 seconds to download 1 byte? After all, what's going on? How to understand that? Wireshark and analyze the traffic or something else?
The page itself is optimized longer - minimized js, css and pictures used where svg and webp background, but no slider or carousel, well, it. It is seen that the downloads decreased, but this does not solve the problem TTFB.
And it is necessary to solve it.