[docs] Add example perfomance Stepper vertical (#24292)

docs/src/pages/components/steppers/steppers.md

@@ -70,6 +70,15 @@ Vertical steppers are designed for narrow screen sizes. They are ideal for mobil
 
 {{"demo": "pages/components/steppers/VerticalLinearStepper.js"}}
 
+### Perfomance
+
+The content of a step is unmounted when closed.
+If you need to make the content available to search engines or render expensive component trees inside your modal while optimizing for interaction responsiveness it might be a good idea to keep the step mounted with:
+
+```jsx
+<StepContent TransitionProps={{ unmountOnExit: false }} />
+```
+
 ## Mobile stepper
 
 This component implements a compact stepper suitable for a mobile device. It has more limited functionality than the vertical stepper. See [mobile steps](https://material.io/archive/guidelines/components/steppers.html#steppers-types-of-steps) for its inspiration.