One of the many performance improvements in the upcoming Solr 1.4 release involves improved filtering performance. Solr 1.4 filters are both faster (anywhere from 30% to 80% faster to calculate intersections, depending on configuration), take less memory (40% smaller), and are more efficiently applied to the query during a search.
In previous Solr releases, filters were applied after the main query and thus had little impact on overall query performance. Filters are now checked in parallel with the query, resulting in greater speedups the fewer documents that match the filters.
Example: Adding a filter that matched 10% of a large index resulted in a 300% performance increase for a dismax query consisting of three words on a single field with proximity boost.