How to properly optimize nginx and php?

Question

Nginx

I took vds for one site on Wordpress with the characteristics of 8 GB of RAM, 1 GB swap, SSD disk, on board: Debian, ispmanager5, Apache MPM-ITK, nginx and PHP-FPM FastCGI.
I set up a site (trilingual, over 10,000 entries, 30,000 images, not including thumbnails, a mega-consuming WPML plugin)
When editing posts, the site started crashing into a 504 nginx
error At the same time, I look at the system load in the panel, I don’t see critical situations there:

That is, something tells me that the server is not used "to the full", most likely there are some software restrictions on the use of resources. Most likely these are limitations of php, nginx, apache
Please tell me what parameters I can "raise" so as not to artificially limit the execution of certain scripts, for example.

Answer 1

Alternatively, throw Apache to hell and use only nginx + php_fpm.

Answer 2

Start by optimizing MySQL. For example using mysqltuner. And even easier, install Bitrix business, there is a performance monitor. You can follow his instructions.
You don't need to go into configs, all parameters are available from ISPmanager.

Answer 3

Did you tune anything on the server or just install the software and that's it?