It always happens like that: when just a couple of users (read: a QA engineer and a customer) are using the software simultaneously, it works fine. However, after release, everything may suddenly become worse as the app is getting popular and attracting users all around the globe.

As the number of requests multiplies, it becomes clear that the server is simply not ready to accommodate a growing audience. When the load reaches the point where every new portion of requests is similar to a DDOS attack, the server eventually burns out and the app crashes.

https://yellow.systems/blog/how-to-build-a-scalable-web-application