When users access moodle they go by default to the personal area, i.e. defaulthomepage is the dashboard.
Simultaneously 1300 people access moodle at the same time, users start complaining about slowness problems in moodle that prevent them from working.
Environment:
Moodle 4.1.11
PHP 8.1
Postgres 15
Redis
We have 3 web servers just for this moodle, with enough resources. After analyzing the problem we observed that when accessing the dashboard load times soar causing extreme slowness. 

The calendar and upcoming events block is active by default in the user dashboard. We noticed that certain database queries are executed too much. 
We reset usage statistics and test for a specific time.

The following query has been launched 90 million times in 4 hours.

We start disabling blocks on the dashboard for all users.

• 200 thousand queries every 10 seconds with the calendar block and upcoming events active.
• 30 thousand every 10 seconds after deactivating the calendar block and next events.

So we can deduce that there is some problem with the calendar block and upcoming events in the dashboard, after disabling it users have been able to work normally in moodle.