How good is Drupal

Any site that dramatically outperforms expectations can have those issues. Most sites don't ramp up that fast though.

All sorts of problems could affect a site that became unexpectedly successful. One common example is getting slashdotted, or linked to from a popular website.

Plan and test.

http://www.workhabit.org/how-do-you-support-500-000-users-drupal
http://www.workhabit.org/database-scalability-and-drupal

You cannot expect any CMS or site to operate the same with 20 users, 5,000, 10,000, 50,000, 200,000 users.

-Steven Peck
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Test site, always start with a test site.
_{Drupal Best Practices Guide -|- Black Mountain}