The most computationally expensive part of searching is score calculation. It may take several seconds for large result sets (tens of thousands of hits).
Zend_Search_Lucene gives the possibility to limit result set size
$currentResultSetLimit = Zend_Search_Lucene::getResultSetLimit();
The default value of 0 means 'no limit'.
It doesn't give the 'best N' results, but only the 'first N' .
 Returned hits are still ordered by score or by the specified order, if given.