Premature optimization may be the root of all evil. But failing to architect an application from the start for reasonable performance and scalability is the premature root of all optimization.
a. What features are going to be added over the course of your application b. What features are going to be popular c. How these features scale, independently of the overall application
Architecting for reasonable performance should always be a goal. Architecting for scalability, however, is often wishful thinking that from my experiences has lead to more harm than good.