johnsmith_123 wrote: when tried to submit more than 50 simultaneous users, I found the CPU resource on the server (Windows server 2016, 4 cores, 16 RAM) becomes 99%
How do you create 50 users? The is a difference between 50 users and 50 concurrent connections. 50 users seldom access LimeSurvey at the same time and e.g. click next at the same time. So you normally test with concurrent connections and not users.
The system load depends on many elements (Hardware, OS,PHP version, Webserver, Databaseserver, PHP-handler of the webserver, php code AKA LimeSurvey, survey content).
You normally would try to benchmark every part of the stack to find bottlenecks. Without knowing the stack, even an educated guess is hard.
Update: I found your bugticket. If others want to add remarks, they can find it here:
bugs.limesurvey.org/view.php?id=16709
