Update doc/development/fe_guide/vue3_migration.md

doc/development/fe_guide/vue3_migration.md

@@ -76,6 +76,9 @@ const FunctionalComp = (props, slots) => {
 }
 ```
 
+NOTE: **Note:**
+It is not recommended to replace stateful components with functional components unless you absolutely need a performance improvement right now. In Vue 3, performance gains for functional components will be negligible.
+
 ## Old slots syntax with `slot` attribute
 
 **Why?**