This is a puzzle to me.
My 2.0.17 forum takes around 20 seconds to open in my browser when I go to the forum as a guest. Logged in it takes only 0.163 seconds.
I'm on a dedicated server using compressed output and persistent connection.
PHP & MySql info:
Seemed to open nice and fast for me, tried with Edge and Chrome and didn't notice any major delay.
First page load: Page created in 1.94 seconds with 16 queries
Second visit: Page created in 0.158 seconds with 10 queries.
I would try to see if there are any slow mysql queries
You can turn on debug mode to see what queries are ran on the page and you can see which parts of the site take the most time to load even as guest.
You can do this by adding the following to your settings.php file
$db_show_debug = true;
I do notice you have Zend OPcache disabled in your PHP config.
Other than that. Going up to PHP 7.3 might make a minor difference in load times under 5%