--- a/services/sync/modules/engines/bookmarks.js
+++ b/services/sync/modules/engines/bookmarks.js
@@ -284,28 +284,30 @@ BookmarksEngine.prototype = {
         url = `<failed: ${ex.toString()}>`;
       }
     }
     return url;
   },
 
   _guidMapFailed: false,
   _buildGUIDMap: function _buildGUIDMap() {
+    let store = this._store;
     let guidMap = {};
     let tree = Async.promiseSpinningly(PlacesUtils.promiseBookmarksTree("", {
       includeItemIds: true
     }));
     function* walkBookmarksTree(tree, parent=null) {
       if (tree) {
         // Skip root node
         if (parent) {
           yield [tree, parent];
         }
         if (tree.children) {
           for (let child of tree.children) {
+            store._sleep(0); // avoid jank while looping.
             yield* walkBookmarksTree(child, tree);
           }
         }
       }
     }
 
     function* walkBookmarksRoots(tree, rootIDs) {
       for (let id of rootIDs) {