How to accurately determine the reason for the site delay on magento2?

Question

Magento

Greetings, there are several sites on magento2,
they are all on the same server, nginx, php-fpm, ssl, one copy of magento2 is used, each domain has its own website, store and storeview. 3 of them are on the default theme - luma, 2 more - on the popular market theme on themeforest.
In general, with the same settings, we have a wild difference in performance, specifically the problem is in the initial load, more precisely, as webpagetest shows, in TTFB - time to first bite.
For clarity - the first site - on the market - is slow, the second - on luma and fast.
Site 1 Site 2





As you can see, the first site has an indecent time before loading the first byte. The logical conclusion is that it's the topic, but a huge number of shops use commercial themes, including market. I take it that if it was unsolvable, no one would use threads like this, right?
The question is how to solve the problem, how to determine exactly what is happening these 5 seconds before loading 1 byte? After all, what is happening? How to understand what? Wireshark and analyze traffic or something else?
The page itself is optimized further - js, css and images are minimized, svg-background and webp are used where possible, not a single slider or carousel, well, everything is just the bare necessities. It can be seen that the content loading time has decreased, but this does not solve the TTFB problem in any way.
And you have to decide.

Answer 1

First, you can try to solve the problem using standard magento tools (without varnish)
devdocs.magento.com/guides/v2.0/config-guide/prod/...
I will say that the theme can go in addition to custom modules, and there no one is immune from suboptimal decisions such as calling objectManager directly, ignoring proxy, ignoring caching of individual blocks, etc.

Answer 2

99.99% of template themes are much slower than default luma because theme developers don't care about code quality and loading speed.