NYCPHP Meetup

[Steve Manes]

Another pertinent question is how many concurrent logged-in users do you
average?  Drupal is actually pretty good about caching pages for
anonymous users because they all see essentially the same output.  But
lots of login accounts creates lots of dynamic overhead, particular with
the database.  While Drupal passes core common data in the environment,
the downside of modules is that they're each responsible for fetching
their own data from the database, where you're at the mercy of the
module developer.

I worked on one Drupal site where there were 26 SQL queries posted for
each page.  Optimizing the database for high traffic and gobs of cache
memory paid the most dividends there.  So is, as another subscriber
noted, profiling the modules you've got running.