User feedback is important than premature optimization

Donald Knuth:

“The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming.”

You must prioritize user feedback first.

Before you worry about handling millions of users, you need to make sure that 100 users even like and want to use your product. Validating user feedback needs to come first.