Docs: add info on batch send performance

Author: medmunds

docs/tips/index.rst

@@ -12,8 +12,4 @@ done with Anymail:
    django_templates
    securing_webhooks
    test_backend
-
-.. TODO:
-..    Working with django-mailer(2)
-..    Sharing backend connections (sessions)
-
+   performance

docs/tips/performance.rst

@@ -0,0 +1,24 @@
+.. _performance:
+
+Batch send performance
+======================
+
+If you are sending batches of hundreds of emails at a time, you can improve
+performance slightly by reusing a single HTTP connection to your ESP's
+API, rather than creating (and tearing down) a new connection for each message.
+
+Most Anymail EmailBackends automatically reuse their HTTP connections when
+used with Django's batch-sending functions :func:`~django.core.mail.send_mass_mail` or
+:meth:`connection.send_messages`. See :ref:`django:topics-sending-multiple-emails`
+in the Django docs for more info and an example.
+
+(The exception is when Anymail wraps an ESP's official Python package, and that
+package doesn't support connection reuse. Django's batch-sending functions will
+still work, but will incur the overhead of creating a separate connection for each
+message sent. Currently, only SparkPost has this limitation.)
+
+If you need even more performance, you may want to consider your ESP's batch-sending
+features. When supported by your ESP, Anymail can send multiple messages with a single
+API call. See :ref:`batch-send` for details, and be sure to check the
+:ref:`ESP-specific info <supported-esps>` because batch sending capabilities vary
+significantly between ESPs.