diff --git a/lib/datalib.php b/lib/datalib.php
index 3509b444495..a9a44da8083 100644
--- a/lib/datalib.php
+++ b/lib/datalib.php
@@ -42,8 +42,13 @@ define('MAX_COURSE_CATEGORIES', 10000);
 
 /**
  * Number of seconds to wait before updating lastaccess information in DB.
+ *
+ * We allow overwrites from config.php, useful to ensure coherence in performance
+ * tests results.
  */
-define('LASTACCESS_UPDATE_SECS', 60);
+if (!defined('LASTACCESS_UPDATE_SECS')) {
+    define('LASTACCESS_UPDATE_SECS', 60);
+}
 
 /**
  * Returns $user object of the main admin user