Issue #2723705 by Wim Leers, catch: Reduce the 200ms BigPipe placeholder interval to 50ms and make it overridable

core/modules/big_pipe/js/big_pipe.js

@@ -89,7 +89,10 @@
     }, interval);
   }
 
-  var interval = 200;
+  // The frequency with which to check for newly arrived BigPipe placeholders.
+  // Hence 50 ms means we check 20 times per second. Setting this to 100 ms or
+  // more would cause the user to see content appear noticeably slower.
+  var interval = drupalSettings.bigPipeInterval || 50;
   // The internal ID to contain the watcher service.
   var timeoutID;