There is no excuse for a modern system to be slow. I’m at a 5,000-people conference this week, and their official networking app is totally overloaded and almost unresponsive.
You might still have legacy systems with scalability issues, but everything you build today should be cloud-native. As a first-class citizen of the cloud, a modern app has access to automatic scaling, monitoring, robustness, and many other features.
Ask the architects building new systems in your organization about how the application will scale. If the answer is that it will scale automatically, good. If the answer is that somebody has to notice response time increasing and manually do anything, you are still building to the old paradigm.