diff --git a/core/utils/toolbox.js b/core/utils/toolbox.js
index e9e7b7ee0..26590a2e6 100644
--- a/core/utils/toolbox.js
+++ b/core/utils/toolbox.js
@@ -99,6 +99,8 @@ Blockly.utils.toolbox.convertToolboxToJSON = function(toolboxDef) {
   }
   // If it is an array of JSON, then it is already in the correct format.
   if (Array.isArray(toolboxDef) && toolboxDef.length && !(toolboxDef[0].nodeType)) {
+    console.warn('Due to some performance issues, defining a toolbox using JSON is not' +
+        ' ready yet. Please define your toolbox using xml.');
     return /** @type {!Array.<Blockly.utils.toolbox.Toolbox>} */ (toolboxDef);
   }