Alain Raap

Alexander did you install Moodle 3.5 with PHP 7.4 version? I once upgraded Moodle 3.5 with PHP 7.3 installed and that didn't work.
Check this thread https://moodle.org/mod/forum/discuss.php?d=378726
When running Moodle 3.5 with PHP 7.2 (or lower), upgrading to Moodle 3.9 runs smoothly

Does it makes a difference to load the admin pages from the outside (internet) or on the inside network? Sometimes loadbalancers (like F5) can be hard to configure with your Moodle configuration (in our case). We had the same kind of problems with long load times in the beginning, it's often difficult to find the bottleneck with these problems. Logging like Splunk or your monitoring tools could be of help. We got timeouts somewhere in the network and found this in the network logfiles (searched on IP address or FQDN).

Just a few more questions from my side, where is your moodledata and how is it connected? NFS (we had in the past) is a very slow network device, so just asking if you use it.
And what is the response time of a page load (I usually check this with the debugger in the browser (F12))? Have you had any issues with caching? Performance boosts are
usually made after tweaking caching of the webserver and / or database.

Hi Emma,

What caching (server) are you using? Or do you use a PHP-FPM server? We have a double server configuration too, and with Moodle 3.9.2 we have a much better performance than with our current version (Moodle 3.5). Automated backup was much faster too. Mysqltuner could be of help too, do you cache the database in memory? That's really a performance improvement.

I have used these articles in the past, maybe you can use them for tuning:

https://medium.com/@sbuckpesch/apache2-and-php-fpm-performance-optimization-step-by-step-guide-1bfecf161534

https://wiki.mikejung.biz/Apache#Event_MPM

https://serversforhackers.com/c/php-fpm-process-management

MPM Event takes some time to figure out how to configure it together with Apache and PHP-FPM
A good performancetest with a baseline is important if you're tweaking with these components for a better performance