diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 46f905ae53d..226912f67e4 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -4846,11 +4846,15 @@ function get_complete_user_data($field, $value, $mnethostid = null, $throwexcept
     // Build the WHERE clause for an SQL query.
     $params = array('fieldval' => $value);
 
-    // Do a case-insensitive query, if necessary.
+    // Do a case-insensitive query, if necessary. These are generally very expensive. The performance can be improved on some DBs
+    // such as MySQL by pre-filtering users with accent-insensitive subselect.
     if (in_array($field, $caseinsensitivefields)) {
         $fieldselect = $DB->sql_equal($field, ':fieldval', false);
+        $idsubselect = $DB->sql_equal($field, ':fieldval2', false, false);
+        $params['fieldval2'] = $value;
     } else {
         $fieldselect = "$field = :fieldval";
+        $idsubselect = '';
     }
     $constraints = "$fieldselect AND deleted <> 1";
 
@@ -4867,6 +4871,10 @@ function get_complete_user_data($field, $value, $mnethostid = null, $throwexcept
         $constraints .= " AND mnethostid = :mnethostid";
     }
 
+    if ($idsubselect) {
+        $constraints .= " AND id IN (SELECT id FROM {user} WHERE {$idsubselect})";
+    }
+
     // Get all the basic user data.
     try {
         // Make sure that there's only a single record that matches our query.