Update How Does It Work

docs/how-does-it-work.rst

@@ -53,7 +53,24 @@ Immutability
 
 In order to give you immutability, ``attrs`` will attach a ``__setattr__`` method to your class that raises a :exc:`attr.exceptions.FrozenInstanceError` whenever anyone tries to set an attribute.
 
-To circumvent that ourselves in ``__init__``, ``attrs`` uses (an aggressively cached) :meth:`object.__setattr__` to set your attributes.
+Depending on whether of not a class is a dict class or a slots class, ``attrs`` uses a different technique to circumvent that limitation in the ``__init__`` method.
+
+Once constructed, frozen instances don't differ in any way from regular ones except that you cannot change its attributes.
+
+
+Dict Classes
+++++++++++++
+
+Dict classes -- i.e. regular classes -- simply assign the value directly into the class' eponymous ``__dict__`` (and there's nothing we can do to stop the user to do the same).
+
+The performance impact is negligible.
+
+
+Slots Classes
++++++++++++++
+
+Slots classes are more complicated.
+Here it uses (an aggressively cached) :meth:`object.__setattr__` to set your attributes.
 This is (still) slower than a plain assignment:
 
 .. code-block:: none
@@ -74,6 +91,10 @@ So on a standard notebook the difference is about 300 nanoseconds (1 second is 1
 It's certainly something you'll feel in a hot loop but shouldn't matter in normal code.
 Pick what's more important to you.
 
-****
 
-Once constructed, frozen instances don't differ in any way from regular ones except that you cannot change its attributes.
+Summary
++++++++
+
+You should avoid to instantiate lots of frozen slots classes (i.e. ``@attr.s(slots=True, frozen=True)``) in performance-critical code.
+
+Frozen dict classes have barely a performance impact, unfrozen slots classes are even *faster* than unfrozen dict classes (i.e. regular classes).