diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c8b39249..8110ed39 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -25,8 +25,9 @@ Changes:
 
 - ``attr.asdict``\ 's ``dict_factory`` arguments is now propagated on recursion.
   `#45 <https://github.com/hynek/attrs/issues/45>`_
-- ``attr.asdict`` and ``attr.has`` are significantly faster.
+- ``attr.asdict``, ``attr.has`` and ``attr.fields`` are significantly faster.
   `#48 <https://github.com/hynek/attrs/issues/48>`_
+  `#51 <https://github.com/hynek/attrs/issues/51>`_
 
 
 ----
diff --git a/src/attr/_funcs.py b/src/attr/_funcs.py
index a1461a35..ba543fe1 100644
--- a/src/attr/_funcs.py
+++ b/src/attr/_funcs.py
@@ -3,7 +3,7 @@ from __future__ import absolute_import, division, print_function
 import copy
 
 from ._compat import iteritems
-from ._make import Attribute, NOTHING, _fast_attrs_iterate
+from ._make import Attribute, NOTHING, fields
 
 
 def asdict(inst, recurse=True, filter=None, dict_factory=dict):
@@ -30,7 +30,7 @@ def asdict(inst, recurse=True, filter=None, dict_factory=dict):
     .. versionadded:: 16.0.0
         *dict_factory*
     """
-    attrs = _fast_attrs_iterate(inst)
+    attrs = fields(inst.__class__)
     rv = dict_factory()
     for a in attrs:
         v = getattr(inst, a.name)
diff --git a/src/attr/_make.py b/src/attr/_make.py
index 55e7fbc0..793707df 100644
--- a/src/attr/_make.py
+++ b/src/attr/_make.py
@@ -1,6 +1,5 @@
 from __future__ import absolute_import, division, print_function
 
-import copy
 import hashlib
 import linecache
 
@@ -417,16 +416,7 @@ def fields(cl):
         raise ValueError("{cl!r} is not an attrs-decorated class.".format(
             cl=cl
         ))
-    return copy.deepcopy(attrs)
-
-
-def _fast_attrs_iterate(inst):
-    """
-    Fast internal iteration over the attr descriptors.
-
-    Using fields to iterate is slow because it involves deepcopy.
-    """
-    return inst.__class__.__attrs_attrs__
+    return attrs
 
 
 def validate(inst):
@@ -440,7 +430,7 @@ def validate(inst):
     if _config._run_validators is False:
         return
 
-    for a in _fast_attrs_iterate(inst):
+    for a in fields(inst.__class__):
         if a.validator is not None:
             a.validator(inst, a, getattr(inst, a.name))
 
@@ -453,7 +443,7 @@ def _convert(inst):
 
     :param inst: Instance of a class with ``attrs`` attributes.
     """
-    for a in _fast_attrs_iterate(inst):
+    for a in inst.__class__.__attrs_attrs__:
         if a.convert is not None:
             setattr(inst, a.name, a.convert(getattr(inst, a.name)))
 
@@ -534,37 +524,38 @@ class Attribute(object):
 
     Plus *all* arguments of :func:`attr.ib`.
     """
-    _attributes = [
-        "name", "default", "validator", "repr", "cmp", "hash", "init",
-        "convert"
-    ]  # we can't use ``attrs`` so we have to cheat a little.
+    __slots__ = ('name', 'default', 'validator', 'repr', 'cmp', 'hash', 'init',
+                 'convert')
 
     _optional = {"convert": None}
 
     def __init__(self, **kw):
-        if len(kw) > len(Attribute._attributes):
+        if len(kw) > len(Attribute.__slots__):
             raise TypeError("Too many arguments.")
-        for a in Attribute._attributes:
+        for a in Attribute.__slots__:
             try:
-                setattr(self, a, kw[a])
+                object.__setattr__(self, a, kw[a])
             except KeyError:
                 if a in Attribute._optional:
-                    setattr(self, a, self._optional[a])
+                    object.__setattr__(self, a, self._optional[a])
                 else:
                     raise TypeError("Missing argument '{arg}'.".format(arg=a))
 
+    def __setattr__(self, name, value):
+        raise AttributeError("can't set attribute")  # To mirror namedtuple.
+
     @classmethod
     def from_counting_attr(cl, name, ca):
         return cl(name=name,
                   **dict((k, getattr(ca, k))
                          for k
-                         in Attribute._attributes
+                         in Attribute.__slots__
                          if k != "name"))
 
 
 _a = [Attribute(name=name, default=NOTHING, validator=None,
                 repr=True, cmp=True, hash=True, init=True)
-      for name in Attribute._attributes]
+      for name in Attribute.__slots__]
 Attribute = _add_hash(
     _add_cmp(_add_repr(Attribute, attrs=_a), attrs=_a), attrs=_a
 )
diff --git a/tests/test_make.py b/tests/test_make.py
index 0529daf7..97c5cb99 100644
--- a/tests/test_make.py
+++ b/tests/test_make.py
@@ -6,9 +6,9 @@ from __future__ import absolute_import, division, print_function
 
 import pytest
 from hypothesis import given
-from hypothesis.strategies import booleans
+from hypothesis.strategies import booleans, sampled_from
 
-from . import simple_attr
+from . import simple_attr, simple_attrs
 from attr import _config
 from attr._compat import PY3
 from attr._make import (
@@ -23,6 +23,8 @@ from attr._make import (
     validate,
 )
 
+attrs = simple_attrs.map(lambda c: Attribute.from_counting_attr('name', c))
+
 
 class TestCountingAttr(object):
     """
@@ -184,6 +186,14 @@ class TestAttributes(object):
         assert "C3()" == repr(C3())
         assert C3() == C3()
 
+    @given(attr=attrs, attr_name=sampled_from(Attribute.__slots__))
+    def test_immutable(self, attr, attr_name):
+        """
+        Attribute instances are immutable.
+        """
+        with pytest.raises(AttributeError):
+            setattr(attr, attr_name, 1)
+
     @pytest.mark.parametrize("method_name", [
         "__repr__",
         "__eq__",
@@ -386,15 +396,6 @@ class TestFields(object):
         """
         assert all(isinstance(a, Attribute) for a in fields(C))
 
-    def test_copies(self, C):
-        """
-        Returns a new list object with new `Attribute` objects.
-        """
-        assert C.__attrs_attrs__ is not fields(C)
-        assert all(new == original and new is not original
-                   for new, original
-                   in zip(fields(C), C.__attrs_attrs__))
-
 
 class TestConvert(object):
     """