Response Time Limits: Article by Jakob Nielsen

This is an old, old, old article.  But still completely relevant today.  You see, human perception hasn’t evolved substantially since 1993, and so neither have our expectations of the response time of the systems we interact with.  Those systems my have substantially evolved as the web became mainstream, and then smartphone apps became ubiquitous.  But our expectations remain the same.  If the system responds to a users action in under 1 second, then the users flow of thought remains unaffected.