What do you test? How much time/effort do you spend testing each of those things? How do you know those will be performance issues?
There's only so much you can do -- making reasonable assumptions, choosing suitable data structures, database indices, testing various workloads, etc. -- in a limited test environment.
You may spend a week optimizing a feature that only 10 people use or that never gets to the point where it becomes an issue. Or you may have something that can't easily be tested at scale, such as various counts or other dynamic data that are determined by complex queries that you may (likely) find you need to cache but don't necessarily know which values will become issues until you start using the system.
We have a test team that runs various test suites(automated) and manual testers that test functionality. They spend their entire working day, every day testing.
There's only so much you can do -- making reasonable assumptions, choosing suitable data structures, database indices, testing various workloads, etc. -- in a limited test environment.
You may spend a week optimizing a feature that only 10 people use or that never gets to the point where it becomes an issue. Or you may have something that can't easily be tested at scale, such as various counts or other dynamic data that are determined by complex queries that you may (likely) find you need to cache but don't necessarily know which values will become issues until you start using the system.