Performance: Skip delegators list generation on every calendar refresh action

plugins/kolab_delegation/kolab_delegation.php

@@ -80,9 +80,11 @@ class kolab_delegation extends rcube_plugin
         }
         // Calendar plugin UI bindings
         else if ($this->rc->task == 'calendar' && empty($_REQUEST['_framed'])) {
+            if ($this->rc->output->type == 'html') {
                 $this->calendar_ui();
             }
         }
+    }
 
     /**
      * Adds Delegation section in Settings