Handling heavy load

Okay, are the puzzles dynamic generated as well or static content? Because you need to start caching content that's not unique to the user. To lighten the load of the database server.

You need to switch out mod_php into php-fpm asap. As of right know it's this:
apache2+php
apache2+php
apache2+php
apache2+php
apache2+php
...
256 times

And go into
apache2
apache2
apache2
apache2
apache2
... (don't know how many you need with Apache)
If it's the same as with Nginx, you need 3, the number of CPU.

php-fpm
php-fpm
php-fpm
php-fpm
php-fpm
... (depending on the amount of free RAM. And how memory hungry your game are.)

The PM pool should be static. Can't go into exact details on how many you will need, as it need to be tuned to the specific application and server specifications. Based on the current load you need to bump PHP higher than before. But move away from mod_php like yesterday! :-)

A little more read about it:
https://tideways.com/profiler/blog/an-in...fpm-tuning