diff --git a/lib/6to5/generation/buffer.js b/lib/6to5/generation/buffer.js
index c9b4e662fe..2fadf5db33 100644
--- a/lib/6to5/generation/buffer.js
+++ b/lib/6to5/generation/buffer.js
@@ -67,32 +67,57 @@ Buffer.prototype.removeLast = function (cha) {
 
 Buffer.prototype.newline = function (i, removeLast) {
   if (this.format.compact) return;
-
-  if (_.isBoolean(i)) {
-    removeLast = i;
-    i = null;
-  }
+  removeLast = removeLast || false;
 
   if (_.isNumber(i)) {
     if (this.endsWith("{\n")) i--;
     if (this.endsWith(util.repeat(i, "\n"))) return;
 
-    for (var j = 0; j < i; j++) {
-      this.newline(null, removeLast);
+    while (i--) {
+      this._newline(removeLast);
     }
     return;
   }
 
+  if (_.isBoolean(i)) {
+    removeLast = i;
+  }
+
+  this._newline(removeLast);
+};
+
+Buffer.prototype._newline = function (removeLast) {
   if (removeLast && this.isLast("\n")) this.removeLast("\n");
 
   this.removeLast(" ");
 
   // remove whitespace if last character was a newline
-  this.buf = this.buf.replace(/\n +$/, "\n");
-
+  this._removeSpacesAfterLastNewline();
   this._push("\n");
 };
 
+/**
+ * If buffer ends with a newline and some spaces after it, trim those spaces.
+ */
+Buffer.prototype._removeSpacesAfterLastNewline = function () {
+  var lastNewlineIndex = this.buf.lastIndexOf('\n');
+  if (lastNewlineIndex === -1)
+    return;
+
+  var index = this.buf.length - 1;
+  while (index > lastNewlineIndex) {
+    if (this.buf[index] !== ' ') {
+      break;
+    }
+
+    index--;
+  }
+
+  if (index === lastNewlineIndex) {
+    this.buf = this.buf.substring(0, index + 1);
+  }
+};
+
 Buffer.prototype.push = function (str, noIndent) {
   if (this._indent && !noIndent && str !== "\n") {
     // we have an indent level and we aren't pushing a newline
diff --git a/lib/6to5/generation/node/index.js b/lib/6to5/generation/node/index.js
index 42daaafaec..179b3015d7 100644
--- a/lib/6to5/generation/node/index.js
+++ b/lib/6to5/generation/node/index.js
@@ -13,7 +13,7 @@ var find = function (obj, node, parent) {
   for (var i = 0; i < types.length; i++) {
     var type = types[i];
 
-    if (t["is" + type](node)) {
+    if (t.is(type, node)) {
       var fn = obj[type];
       result = fn(node, parent);
       if (result != null) break;
@@ -28,13 +28,11 @@ function Node(node, parent) {
   this.node   = node;
 }
 
-Node.prototype.isUserWhitespacable = function () {
-  return t.isUserWhitespacable(this.node);
+Node.isUserWhitespacable = function (node) {
+  return t.isUserWhitespacable(node);
 };
 
-Node.prototype.needsWhitespace = function (type) {
-  var parent = this.parent;
-  var node   = this.node;
+Node.needsWhitespace = function (node, parent, type) {
   if (!node) return 0;
 
   if (t.isExpressionStatement(node)) {
@@ -51,18 +49,15 @@ Node.prototype.needsWhitespace = function (type) {
   return lines || 0;
 };
 
-Node.prototype.needsWhitespaceBefore = function () {
-  return this.needsWhitespace("before");
+Node.needsWhitespaceBefore = function (node, parent) {
+  return Node.needsWhitespace(node, parent, "before");
 };
 
-Node.prototype.needsWhitespaceAfter = function () {
-  return this.needsWhitespace("after");
+Node.needsWhitespaceAfter = function (node, parent) {
+  return Node.needsWhitespace(node, parent, "after");
 };
 
-Node.prototype.needsParens = function () {
-  var parent = this.parent;
-  var node   = this.node;
-
+Node.needsParens = function (node, parent) {
   if (!parent) return false;
 
   if (t.isNewExpression(parent) && parent.callee === node) {
@@ -77,10 +72,7 @@ Node.prototype.needsParens = function () {
   return find(parens, node, parent);
 };
 
-Node.prototype.needsParensNoLineTerminator = function () {
-  var parent = this.parent;
-  var node   = this.node;
-
+Node.needsParensNoLineTerminator = function (node, parent) {
   if (!parent) return false;
 
   // no comments
@@ -100,17 +92,18 @@ Node.prototype.needsParensNoLineTerminator = function () {
   return false;
 };
 
-_.each(Node.prototype, function (fn, key) {
-  Node[key] = function (node, parent) {
-    var n = new Node(node, parent);
-
+_.each(Node, function (fn, key) {
+  Node.prototype[key] = function () {
     // Avoid leaking arguments to prevent deoptimization
-    var skipCount = 2;
-    var args = new Array(arguments.length - skipCount);
+    var args = new Array(arguments.length + 2);
+
+    args[0] = this.node;
+    args[1] = this.parent;
+
     for (var i = 0; i < args.length; i++) {
-      args[i] = arguments[i + 2];
+      args[i + 2] = arguments[i];
     }
 
-    return n[key].apply(n, args);
+    return Node[key].apply(null, args);
   };
 });
diff --git a/lib/6to5/generation/whitespace.js b/lib/6to5/generation/whitespace.js
index c7dda860da..aa0993bc2a 100644
--- a/lib/6to5/generation/whitespace.js
+++ b/lib/6to5/generation/whitespace.js
@@ -1,10 +1,38 @@
 module.exports = Whitespace;
 
-var _ = require("lodash");
+var _    = require("lodash");
+
+/**
+ * Returns `i`th number from `base`, continuing from 0 when `max` is reached.
+ * Useful for shifting `for` loop by a fixed number but going over all items.
+ *
+ * @param {Number} i Current index in the loop
+ * @param {Number} base Start index for which to return 0
+ * @param {Number} max Array length
+ * @returns {Number} shiftedIndex
+ */
+function getLookupIndex(i, base, max) {
+  i += base;
+
+  if (i >= max)
+    i -= max;
+
+  return i;
+}
 
 function Whitespace(tokens, comments) {
   this.tokens = _.sortBy(tokens.concat(comments), "start");
-  this.used   = [];
+  this.used   = {};
+
+  // Profiling this code shows that while generator passes over it, indexes
+  // returned by `getNewlinesBefore` and `getNewlinesAfter` are always increasing.
+
+  // We use this implementation detail for an optimization: instead of always
+  // starting to look from `this.tokens[0]`, we will start `for` loops from the
+  // previous successful match. We will enumerate all tokens—but the common
+  // case will be much faster.
+
+  this._lastFoundIndex = 0;
 }
 
 Whitespace.prototype.getNewlinesBefore = function (node) {
@@ -13,13 +41,17 @@ Whitespace.prototype.getNewlinesBefore = function (node) {
   var tokens = this.tokens;
   var token;
 
-  for (var i = 0; i < tokens.length; i++) {
+  for (var j = 0; j < tokens.length; j++) {
+    // optimize for forward traversal by shifting for loop index
+    var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
     token = tokens[i];
 
     // this is the token this node starts with
     if (node.start === token.start) {
       startToken = tokens[i - 1];
       endToken = token;
+
+      this._lastFoundIndex = i;
       break;
     }
   }
@@ -33,13 +65,17 @@ Whitespace.prototype.getNewlinesAfter = function (node) {
   var tokens = this.tokens;
   var token;
 
-  for (var i = 0; i < tokens.length; i++) {
+  for (var j = 0; j < tokens.length; j++) {
+    // optimize for forward traversal by shifting for loop index
+    var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
     token = tokens[i];
 
     // this is the token this node ends with
     if (node.end === token.end) {
       startToken = token;
       endToken = tokens[i + 1];
+
+      this._lastFoundIndex = i;
       break;
     }
   }
@@ -60,15 +96,14 @@ Whitespace.prototype.getNewlinesAfter = function (node) {
 Whitespace.prototype.getNewlinesBetween = function (startToken, endToken) {
   var start = startToken ? startToken.loc.end.line : 1;
   var end   = endToken.loc.start.line;
-
   var lines = 0;
 
   for (var line = start; line < end; line++) {
-    if (!_.contains(this.used, line)) {
-      this.used.push(line);
+    if (typeof this.used[line] === 'undefined') {
+      this.used[line] = true;
       lines++;
     }
   }
 
   return lines;
-};
+};
\ No newline at end of file
diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index c00e9506af..75270807bf 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -7,14 +7,25 @@ t.NATIVE_TYPE_NAMES = ["Array", "Object", "Number", "Boolean", "Date", "Array",
 
 //
 
-var addAssert = function (type, is) {
+/**
+ * Registers `is[Type]` and `assert[Type]` generated functions for a given `type`.
+ * Pass `skipAliasCheck` to force it to directly compare `node.type` with `type`.
+ *
+ * @param {String} type
+ * @param {Boolean?} skipAliasCheck
+ */
+function registerType(type, skipAliasCheck) {
+  var is = t["is" + type] = function (node, opts) {
+    return t.is(type, node, opts, skipAliasCheck);
+  };
+
   t["assert" + type] = function (node, opts) {
     opts = opts || {};
     if (!is(node, opts)) {
       throw new Error("Expected type " + JSON.stringify(type) + " with option " + JSON.stringify(opts));
     }
   };
-};
+}
 
 t.STATEMENT_OR_BLOCK_KEYS = ["consequent", "body"];
 
@@ -22,14 +33,59 @@ t.STATEMENT_OR_BLOCK_KEYS = ["consequent", "body"];
 
 t.VISITOR_KEYS = require("./visitor-keys");
 
-_.each(t.VISITOR_KEYS, function (keys, type) {
-  var is = t["is" + type] = function (node, opts) {
-    return node && node.type === type && t.shallowEqual(node, opts);
-  };
+t.ALIAS_KEYS = require("./alias-keys");
 
-  addAssert(type, is);
+t.FLIPPED_ALIAS_KEYS = {};
+
+_.each(t.VISITOR_KEYS, function (keys, type) {
+  registerType(type, true);
 });
 
+_.each(t.ALIAS_KEYS, function (aliases, type) {
+  _.each(aliases, function (alias) {
+    var types = t.FLIPPED_ALIAS_KEYS[alias] = t.FLIPPED_ALIAS_KEYS[alias] || [];
+    types.push(type);
+  });
+});
+
+_.each(t.FLIPPED_ALIAS_KEYS, function (types, type) {
+  t[type.toUpperCase() + "_TYPES"] = types;
+  registerType(type, false);
+});
+
+/**
+ * Returns whether `node` is of given `type`.
+ * For better performance, use this instead of `is[Type]` when `type` is unknown.
+ * Optionally, pass `skipAliasCheck` to directly compare `node.type` with `type`.
+ *
+ * @param {String} type
+ * @param {Node} node
+ * @param {Object?} opts
+ * @param {Boolean?} skipAliasCheck
+ * @returns {Boolean} isOfType
+ */
+t.is = function (type, node, opts, skipAliasCheck) {
+  if (!node) return;
+
+  var typeMatches = (type === node.type);
+
+  if (!typeMatches && !skipAliasCheck) {
+    var aliases = t.FLIPPED_ALIAS_KEYS[type];
+
+    if (typeof aliases !== 'undefined')
+      typeMatches = aliases.indexOf(node.type) > -1;
+  }
+
+  if (!typeMatches) {
+    return false;
+  }
+
+  if (typeof opts !== 'undefined')
+    return t.shallowEqual(node, opts);
+
+  return true;
+};
+
 //
 
 t.BUILDER_KEYS = _.defaults(require("./builder-keys"), t.VISITOR_KEYS);
@@ -45,29 +101,6 @@ _.each(t.BUILDER_KEYS, function (keys, type) {
   };
 });
 
-//
-
-t.ALIAS_KEYS = require("./alias-keys");
-
-t.FLIPPED_ALIAS_KEYS = {};
-
-_.each(t.ALIAS_KEYS, function (aliases, type) {
-  _.each(aliases, function (alias) {
-    var types = t.FLIPPED_ALIAS_KEYS[alias] = t.FLIPPED_ALIAS_KEYS[alias] || [];
-    types.push(type);
-  });
-});
-
-_.each(t.FLIPPED_ALIAS_KEYS, function (types, type) {
-  t[type.toUpperCase() + "_TYPES"] = types;
-
-  var is = t["is" + type] = function (node, opts) {
-    return node && types.indexOf(node.type) >= 0 && t.shallowEqual(node, opts);
-  };
-
-  addAssert(type, is);
-});
-
 /**
  * Description
  *
@@ -139,17 +172,17 @@ t.toSequenceExpression = function (nodes, scope) {
 //
 
 t.shallowEqual = function (actual, expected) {
-  var same = true;
+  var keys = Object.keys(expected);
+  var key;
 
-  if (expected) {
-    _.each(expected, function (val, key) {
-      if (actual[key] !== val) {
-        return same = false;
-      }
-    });
+  for (var i = 0; i < keys.length; i++) {
+    key = keys[i];
+
+    if (actual[key] !== expected[key])
+      return false;
   }
 
-  return same;
+  return true;
 };
 
 /**