diff --git a/lark/parse_tree_builder.py b/lark/parse_tree_builder.py
index de88cd1..c56483b 100644
--- a/lark/parse_tree_builder.py
+++ b/lark/parse_tree_builder.py
@@ -61,7 +61,10 @@ class ChildFilter:
         filtered = []
         for i, to_expand in self.to_include:
             if to_expand:
-                filtered += children[i].children
+                if filtered:
+                    filtered += children[i].children
+                else:   # Optimize for left-recursion
+                    filtered = children[i].children
             else:
                 filtered.append(children[i])
 
diff --git a/lark/tree.py b/lark/tree.py
index a9d4670..68eaf42 100644
--- a/lark/tree.py
+++ b/lark/tree.py
@@ -11,7 +11,7 @@ from .utils import inline_args
 class Tree(object):
     def __init__(self, data, children):
         self.data = data
-        self.children = list(children)
+        self.children = children
 
     def __repr__(self):
         return 'Tree(%s, %s)' % (self.data, self.children)