One of the common (mostly unfounded) complaints about Drupal has been, "Drupal is slow." You want to hire a developer who understands Drupal's built in caching system, and what its limitations are. For example, Drupal 6's block cache will not appreciably speed up the page if the user is logged in.

Ask your candidate to recommend some additional solutions to speed up Drupal's caching. These could include the Boost module, Varnish, Squid, Memcache or Pressflow. Ask if they've ever run into issues with Drupal's cache.