doc: add a note about patch performance

src/content/docs/code-push/patch.mdx

@@ -87,3 +87,22 @@ need to put `flutter build` arguments after a `--` separator. For example:
 `shorebird patch android -- --dart-define="foo=bar"` will define the `"foo"` environment
 variable inside Dart as you might have done with `flutter build` directly.
 :::
+
+
+### Patch Performance
+
+Android: Patching an application on Android has no effect on performance.
+
+iOS: Patching an application on iOS normally does not affect an application
+performance. However the patching mechanism on iOS is different from Android.
+Unchanged code runs as normal (on the CPU), however changed (or added) code will
+run in a Dart interpreter (which is slower than the CPU)  Normally this change
+is undetectable, but if you are changing particularly performance-sensitive code
+(e.g. code for processing images, or large data that is written in Dart) you may
+see a performance difference after patching. If you ever see unexpected
+performance changes when patching on iOS, please [contact
+us](mailto:[email protected]) we would love to work with you to understand
+and fix!
+
+You can always test your patches before sending them to users using by
+[staging patches](/staging-patches)