Okay, after all imported:
And:
Nginx optimize
And what happens if you disable some ( or all ) of the WP plugins?
Like Wordfence, All in one security. Disable them and check again. How many plugin's do you have active? These security plugins were famous for eating recources, but maybe that changed over the years.
I have 11 WP sites on that server with ngninx. Each WP site is using about 20 plugins and Generatepress theme. Caching done with Fastcgi no problems.
Can you test disabling plugins and see if that helps? You could also try to install the Query Monitor plugin ( by John Blackbourn) to see if there are heavy queries, Just some suggestions.
10-15 plugin on 1 site.
I deleted 3 site, moved to old OLS server. This is terrible... Nginx is only good for Proxy to Apache, I think Apache not eat so much memory like Nginx...
But did you test disabling all plugins ?
I find it a bit to easy to say that ngninx is terrible without looking further. Even when you use just 3 plugins, when one of them is scripted poor, the result can be slow site's.
I also run ngninx, but don't have those problems. It is not so hard to make WP slow with the wrong plugins/themes. The opposite is also true 
Anyway, I wish you luck with testing
CanΒ΄t be a bad plugin, because before on OLS server max use of ram is 2GB. Same good plugins, always updated.
JimmyBobsa Can you share your Nginx settings?
JimmyBobsa what version of nginx are you using? and what type of database?
You don't provide solutions just by showing off. I think it's not good enough to share in this forum.
If you want to share, give us your settings. I think that is a good thing in the discussion forum.
ady0651 If you want to share, give us your settings
Fair enough
I didn't show the screen for showing off, just to let Tommyboy see that ngninx is not crap per definition
All settings are standard. Things I did -
Follow this guide here
https://www.aapanel.com/forum/d/593-how-to-enable-fastcgi-cache-for-wordpress-with-aapanel
In WP are NO caching plugins, but I use plugins autoptimize, Async JavaScript. Installed on VPS- Redis and fastcgi. Also the plugin WPS Hide Login to prevent bruteforce as much as possible. At the moment using PHP7.3 because I find that ( in real world ) it is the fastest. With 7.4 and 8 I get worse results
Theme only use GeneratePress pro because it is fast out of the box with lots of options.
VPS is from Hetzner Germany, I think they are speedy at low cost.
So, no special configurations. Pretty much all standard.
JimmyBobsa did you found the way to have fastcgi-cache in multiple sites ?
JimmyBobsa Well like this is good enough in the discussion. Oh, so I'm using nginx version 1.17, it seems quite ok, because on nginx versions 1.19 and 1.21 it takes up a lot of memory, I also experienced it. Maybe have to try the version below again, like you bro.
1.21 using like on photos. Eat a lot of memory yes... I go try switch lower, but is secure?
- Edited
Stealth I didn't change the nginx version and stick with 1.21, but I minimized max_childern in the php optimization settings. On the initial setup of my 2/4gb vps max_childern 80 and I changed to 50 it pretty much reduces memory consumption.
Referring to the following website https://serverpilot.io/docs/how-to-change-the-php-fpm-max_children-setting
20 = 20,000 visitors means that by setting 50 = maybe 50,000 visitors
lucius100 did you found the way to have fastcgi-cache in multiple sites ?
Nope, they all share the same cache directory wich kinda sucks of course. I tried different way's, but no succes yet 
ady0651 On the initial setup of my 2/4gb vps max_childern 80
I also have that on 80, I didn't change that. But good to know, thanks!
Well, first of all, I'll admit a mistake here first, I was lazy, I said before that I was going to do a stress test to compare the comparison of before and after stress test with php extensions turned on: redis /memcached/mongodb/opcache, but I didn't do it, and I'll try to do it in the next working week. So that I can give you a statement.