If you don't have intelligent queries and a proper cache system. At least. You will quickly crash your database or saturate your traffic. It can even hurt small traffic websites (I remember coding a webchat with Mysql and crashing my database with only 5 people chatting).
Are many websites running into speed or scalability issues?
I see programmers in other areas spend a lot of man-hours optimizing code that isn't a meaningful bottleneck. Is the same thing happening here?