📝 Tweak docs about performance with ORJONResponse

docs/en/docs/advanced/custom-response.md

@@ -21,7 +21,11 @@ For example, if you are squeezing performance, you can install and use <a href="
 
 Import the `Response` class (sub-class) you want to use and declare it in the *path operation decorator*.
 
-For large responses, returning a `Response` directly is much faster than returning a dictionary and having FastAPI inspect every item inside. However, you must be certain that the dictionary you pass to the Response is json serializable (or subclass the JSONResponse class and add your own custom serialization).
+For large responses, returning a `Response` directly is much faster than returning a dictionary.
+
+This is because by default, FastAPI will inspect every item inside and make sure it is serializable with JSON, using the same [JSON Compatible Encoder](../tutorial/encoder.md){.internal-link target=_blank} explained in the tutorial. This is what allows you to return **arbitrary objects**, for example database models.
+
+But if you are certain that the content that you are returning is **serializable with JSON**, you can pass it directly to the response class and avoid the extra overhead that FastAPI would have by passing your return content through the `jsonable_encoder` before passing it to the response class.
 
 ```Python hl_lines="2  7"
 {!../../../docs_src/custom_response/tutorial001b.py!}