diff --git a/src/wp-includes/vars.php b/src/wp-includes/vars.php
index c903101997..d894884c48 100644
--- a/src/wp-includes/vars.php
+++ b/src/wp-includes/vars.php
@@ -120,9 +120,17 @@ $is_iis7 = $is_IIS && intval( substr( $_SERVER['SERVER_SOFTWARE'], strpos( $_SER
 /**
  * Test if the current browser runs on a mobile device (smart phone, tablet, etc.)
  *
+ * @staticvar bool $is_mobile
+ *
  * @return bool
  */
 function wp_is_mobile() {
+	static $is_mobile = null;
+
+	if ( isset( $is_mobile ) ) {
+		return $is_mobile;
+	}
+
 	if ( empty($_SERVER['HTTP_USER_AGENT']) ) {
 		$is_mobile = false;
 	} elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile') !== false // many mobile devices (all iPhone, iPad, etc.)