Well, my experience is that web apps are most often bottlenecked by the database speed, because almost any other problem can be solved with money (and not even much of it).

I've seen databases bottleneck on lots of different resources, even some virtual ones (unexpected serializing). But I'm very suspicious of people claiming that one must write webapps in low level languages "because speed". (And yes, I know there exist problems out there where this is true, the same way that there exist people out there that've won the lottery.)

It depends on the web app. My experience is that being bottlenecked on the database, while natural, is so crippling to scaling that extensive caching is used to prevent the majority of requests from touching the database.