diff --git a/lib/accesslib.php b/lib/accesslib.php
index f9e4e71cfdd..dbb20318e70 100644
--- a/lib/accesslib.php
+++ b/lib/accesslib.php
@@ -572,8 +572,21 @@ function is_siteadmin($user_or_id = null) {
         $userid = $user_or_id;
     }
 
+    // Because this script is called many times (150+ for course page) with
+    // the same parameters, it is worth doing minor optimisations. This static
+    // cache stores the value for a single userid, saving about 2ms from course
+    // page load time without using significant memory. As the static cache
+    // also includes the value it depends on, this cannot break unit tests.
+    static $knownid, $knownresult, $knownsiteadmins;
+    if ($knownid === $userid && $knownsiteadmins === $CFG->siteadmins) {
+        return $knownresult;
+    }
+    $knownid = $userid;
+    $knownsiteadmins = $CFG->siteadmins;
+
     $siteadmins = explode(',', $CFG->siteadmins);
-    return in_array($userid, $siteadmins);
+    $knownresult = in_array($userid, $siteadmins);
+    return $knownresult;
 }
 
 /**
diff --git a/lib/tests/accesslib_test.php b/lib/tests/accesslib_test.php
index 847724d754f..03a40c6e946 100644
--- a/lib/tests/accesslib_test.php
+++ b/lib/tests/accesslib_test.php
@@ -123,7 +123,7 @@ class accesslib_testcase extends advanced_testcase {
      * @return void
      */
     public function test_is_siteadmin() {
-        global $DB;
+        global $DB, $CFG;
 
         $this->resetAfterTest();
 
@@ -145,6 +145,20 @@ class accesslib_testcase extends advanced_testcase {
                 $this->assertFalse(is_siteadmin(null));
             }
         }
+
+        // Change the site admin list and check that it still works with
+        // multiple admins. We do this with userids only (not real user
+        // accounts) because it makes the test simpler.
+        $before = $CFG->siteadmins;
+        set_config('siteadmins', '666,667,668');
+        $this->assertTrue(is_siteadmin(666));
+        $this->assertTrue(is_siteadmin(667));
+        $this->assertTrue(is_siteadmin(668));
+        $this->assertFalse(is_siteadmin(669));
+        set_config('siteadmins', '13');
+        $this->assertTrue(is_siteadmin(13));
+        $this->assertFalse(is_siteadmin(666));
+        set_config('siteadmins', $before);
     }
 
     /**