I have written a lot of database-driven web applications in dynamic languages, and the database has never been the bottleneck. It has always been in the application code.
(Actually, not true, when I worked for $LARGE_ADVERTISING_COMPANY, we checked the state of the session in the database for every request. This put an enormous load on the database, and made every page load take at least two seconds waiting for locks. No, I did not design any part of this system, nor was I allowed to fix it; so it doesn't count :)
