gistfile1.php

<?php
for ($i=1; $i <= 10; $i++) {

$start = microtime(true);

for ($ii=0; $ii < 5000; $ii++) {

  $tids1 = $tids2 = $tids3 = array();
  for ($iii=0; $iii < 3; $iii++) {
    $tids1[] = rand(500, 1000000);
    $tids2[] = rand(500, 1000000);
    $tids3[] = rand(500, 1000000);
  }

  //Stuff the randomly selected tids into an array compatible with db_query
  $args = array(
    ":tids1" => $tids1,
    ":tids2" => $tids2,
    ":tids3" => $tids3,
  );

  //Get a ranked list of the 5 nodes which have the most matches
  $query = db_query(
    "SELECT node.nid nid
    FROM node
    LEFT JOIN field_data_field_tags
    ON node.nid=field_data_field_tags.entity_id
    WHERE field_data_field_tags.field_tags_tid
    IN (:tids1)

    UNION ALL SELECT node.nid nid
    FROM node
    LEFT JOIN field_data_field_test
    ON node.nid=field_data_field_test.entity_id
    WHERE field_data_field_test.field_test_tid
    IN (:tids2)

    UNION ALL SELECT node.nid nid
    FROM node
    LEFT JOIN field_data_field_test2
    ON node.nid=field_data_field_test2.entity_id
    WHERE field_data_field_test2.field_test2_tid
    IN (:tids3)",
    $args
  );

  $results = $query->fetchAll();
  $result_array = array();
  foreach ($results as $result) {
    $result_array[] = $result->nid;
  }
  arsort(array_count_values($result_array));
  $result_array = array_slice($result_array, 0, 5, true);

}

print "Run $i: " . round(microtime(true) - $start, 3) . ' seconds' . "\n";
}