I have a D7 site with around 300,000 nodes. I am passing this function an array of 300-500 nids. Sometimes the function runs in under a second. Sometimes it takes 40 seconds or more! I can't figure out what causes the slow runs. How can I get the query consistent and fast?

function _load_field_values_for_nids($field_name, $nids)
{
  $field_info = field_info_field($field_name);
  $field_id = $field_info['id'];

  $map = array();
  $nodes_without_required_field = [];

  // Load up the properties from the node table.
  $sql = 'SELECT * FROM {node} WHERE nid IN (:nids)';
  $nodes = db_query($sql, array(':nids' => $nids))->fetchAllAssoc('nid');

  // Attach the single field to all nodes.
  field_attach_load('node', $nodes, FIELD_LOAD_CURRENT, array('field_id' => $field_id));

  foreach ($nodes as $nid => $node) {

    if (isset($node->{$field_name}['und'])) {
      $map[$node->nid] = $node->{$field_name}['und'][0]['value'];
    } else {
      $nodes_without_required_field[] = $node->nid;
    }
  }

  $absent_count = count($nodes_without_required_field);
  if ($absent_count > 0) {
    watchdog("vwyw_load_fields", $absent_count." nodes did not have field, \"".$field_name."\" ");
  }

  return $map;
}

I noticed this SIX-SECOND query on the main /admin/content page (which took 12+seconds to load):

Here is the expanded XHProf output regarding that Count query:

I think this might be a mysql configuration issue rather that the particulars of a query. For instance, I think this query should run very quickly but it takes a second and a half:

The next time (same page load) it took 120ms and the time after that, 4.8sec!

UPDATE

Here is the XHProf output sorted by duration for a simple admin page that calls the function: