Why is default Luma theme so slow (Time to interactive) on PageSpeed insight?

I had bad result on Google Pagespeed Insight on small Magento Websites, using default theme and modifying only little stuff, plus adding static content to homepages.

And Pagespeed Insight rating are bad, more on mobile.

So I installed an empty 2.2.8 shop (no categorie, no product) to try to find out what I was doing wrong.

I tested with GTMetrix and get an A (99%) on PageSpeed (so I guess it’s not a configuration problem)

JS and CSS are minified, even HTML. I even installed a Varnish to be sure !

But PageSpeed Insight ratings are as bad as with modified Luma. (around 38-40)

https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Ftestmg2.g-softs.net%2F (the link might not be valid in time, it’s just a test website)

So does anyone knows why default template got so bad ratings ?

Is there a way to improve it easily (I’m more of a backend than a frontend dev) ?

Do you know an ultra basic templates with good PageSpeed ratings ?

Thanks in advance !