Dear All,
The Performance of moodle 3.5 Slow Down when 20 or more users login at a time.
Please help
Any Moodle version, for that matter any software which server a number of users, will slow down if there is not sufficient resources. It is also possible that something is broken, but before going that path you have to make sure that the resources are sufficient. That is your starting point - and of course the forum documentation: https://docs.moodle.org/en/Performance and https://docs.moodle.org/en/Performance_FAQ.