Search based on user profiles in Drupal 7
Searching with Drupal core is not very performative because you are doing search queries in a mysql database.

Apache Solr is much more performative and includes a much greater feature set for searching Drupal content: http://drupal.org/project/apachesolr

If you want to limit search results just to users you can use the solr module above plus Facet API http://drupal.org/project/facetapi and only expose the facets for the content authors. You can also create a view using Solr Views that will return the author in the search results.