数据库何时以及如何减慢网站速度？

I have read a book by O'Reilly that states that 70-80% of the performance of a site can be optimized via the front-end. That may mean that database queries may account to less than 20 or 30% of a site's performance. However, I have seen huge websites like Facebook and Twitter dedicate so much time to optimizing queries (via query caching, normalizations, etc). When does the database become so critical for a site's performance that it can account to more than the aforementioned percentage? Oh, and by performance I mean in the context of speed, loading speed in particular.I have read a book by O'Reilly that states that