800 candidates taking an on-line exam at the same time is heavy. The general advice is here: https://docs.moodle.org/en/Performance_recommendations. I am not sure how far that applies to you because your users are on mobile devices. See https://moodle.org/mod/forum/view.php?id=7798.
The general approach should be still valid: You need to monitor the server closely when it gets overloaded, identify the component which hits its limits - CPU, RAM, disk I/O, network. Then treat it in a targeted way. It could mean a configuration change, could also mean adding resources.
The general approach should be still valid: You need to monitor the server closely when it gets overloaded, identify the component which hits its limits - CPU, RAM, disk I/O, network. Then treat it in a targeted way. It could mean a configuration change, could also mean adding resources.
Read the many similar discussions in this forum, https://moodle.org/mod/forum/discuss.php?d=411464 for example. And make use of the advanced search https://moodle.org/mod/forum/search.php?id=5.