DOCSP-18812: Change Streams Performance (#10)

* DOCSP-188812: Change Streams Performance info

Author: Chris Cho

source/source-connector/fundamentals/change-streams.txt

@@ -21,22 +21,16 @@ used in a {+mkc+} source connector.
 Change Streams
 --------------
 
-Change streams are a feature of MongoDB that allow you to receive real-time
-updates on data changes. Change streams return
-**change event documents**. A change event document is a document in your
-**oplog** that contains idempotent instructions to recreate a change that
-occurred in your MongoDB deployment as well as the metadata related to that
-change.
+Change streams are a MongoDB feature that allow you to receive real-time
+updates on data changes. Change streams return **change event documents**.
 
-The oplog is a special collection in MongoDB that keeps track of all changes
-within a MongoDB replica set. Change streams help you use the change event data
-stored in the oplog without you having to learn details about how the oplog
-works.
+A change event document contains idempotent instructions to describe a change
+that occurred in your MongoDB deployment and metadata related to that change.
+Change event documents are generated from data in the :term:`oplog`.
 
-.. important:: You must have a replica set or a sharded cluster
+.. important:: Change streams only run on MongoDB replica sets and sharded clusters
 
-   Change streams are available for replica sets and sharded clusters. A
-   standalone MongoDB instance cannot produce a change stream.
+   A standalone MongoDB instance cannot produce a change stream.
 
 To view a list of all configuration options for change streams, see the
 :ref:`<source-configuration-change-stream>` page.
@@ -88,6 +82,18 @@ descriptions of all fields,
    option. For more information, see the :ref:`<source-configuration-change-stream>`
    page.
 
+Performance
+~~~~~~~~~~~
+
+The oplog is a special capped collection which cannot use indexes.  For more
+information on this limitation, see
+:manual:`Change Streams Production Recommendations </administration/change-streams-production-recommendations/#indexes>`.
+
+If you need to improve change stream performance, use a faster disk for
+your MongoDB cluster and increase the size of your WiredTiger cache. To
+learn how to set your WiredTiger cache, see the guide on the
+:manual:`WiredTiger Storage Engine </core/wiredtiger/#memory-use>`.
+
 Source Connectors
 -----------------