doc: add warning about complex global state performance

Author: kbond

docs/index.rst

@@ -1484,7 +1484,9 @@ Global State
 ~~~~~~~~~~~~
 
 If you have an initial database state you want for all tests, you can set this in the config of the bundle. Accepted
-values are: stories as service, "global" stories and invokable services.
+values are: stories as service, "global" stories and invokable services. Global state is loaded before each using
+the ``ResetDatabase`` trait. If you are using `DamaDoctrineTestBundle`_, it is only loaded once for the entire
+test suite.
 
 .. configuration-block::
 
@@ -1508,6 +1510,10 @@ values are: stories as service, "global" stories and invokable services.
 
     The :ref:`ResetDatabase <enable-foundry-in-your-testcase>` trait is required when using global state.
 
+.. warning::
+
+    Be aware that a complex global state could slow down your test suite.
+
 PHPUnit Data Providers
 ~~~~~~~~~~~~~~~~~~~~~~