diff --git a/CHANGES.rst b/CHANGES.rst
index 1fbbed87..a57a97d2 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -47,6 +47,11 @@ Unreleased
 -   Fix type annotation for ``errorhandler`` decorator. :issue:`4295`
 -   Revert a change to the CLI that caused it to hide ``ImportError``
     tracebacks when importing the application. :issue:`4307`
+-   ``app.json_encoder`` and ``json_decoder`` are only passed to
+    ``dumps`` and ``loads`` if they have custom behavior. This improves
+    performance, mainly on PyPy. :issue:`4349`
+-   Clearer error message when ``after_this_request`` is used outside a
+    request context. :issue:`4333`
 
 
 Version 2.0.2
diff --git a/docs/server.rst b/docs/server.rst
index 446d5be8..77b25abb 100644
--- a/docs/server.rst
+++ b/docs/server.rst
@@ -80,27 +80,34 @@ messages:
 Either identify and stop the other program, or use
 ``flask run --port 5001`` to pick a different port.
 
-You can use ``netstat`` to identify what process id is using a port,
-then use other operating system tools stop that process. The following
-example shows that process id 6847 is using port 5000.
+You can use ``netstat`` or ``lsof`` to identify what process id is using
+a port, then use other operating system tools stop that process. The
+following example shows that process id 6847 is using port 5000.
 
 .. tabs::
 
-    .. group-tab:: Linux/Mac
+    .. tab:: ``netstat`` (Linux)
 
         .. code-block:: text
 
             $ netstat -nlp | grep 5000
             tcp 0 0 127.0.0.1:5000 0.0.0.0:* LISTEN 6847/python
 
-    .. group-tab:: Windows
+    .. tab:: ``lsof`` (macOS / Linux)
+
+        .. code-block:: text
+
+            $ lsof -P -i :5000
+            Python 6847 IPv4 TCP localhost:5000 (LISTEN)
+
+    .. tab:: ``netstat`` (Windows)
 
         .. code-block:: text
 
             > netstat -ano | findstr 5000
             TCP 127.0.0.1:5000 0.0.0.0:0 LISTENING 6847
 
-MacOS Monterey and later automatically starts a service that uses port
+macOS Monterey and later automatically starts a service that uses port
 5000. To disable the service, go to System Preferences, Sharing, and
 disable "AirPlay Receiver".
 
diff --git a/src/flask/ctx.py b/src/flask/ctx.py
index 05ba4938..b3687ba7 100644
--- a/src/flask/ctx.py
+++ b/src/flask/ctx.py
@@ -130,7 +130,15 @@ def after_this_request(f: AfterRequestCallable) -> AfterRequestCallable:
 
     .. versionadded:: 0.9
     """
-    _request_ctx_stack.top._after_request_functions.append(f)
+    top = _request_ctx_stack.top
+
+    if top is None:
+        raise RuntimeError(
+            "This decorator can only be used when a request context is"
+            " active, such as within a view function."
+        )
+
+    top._after_request_functions.append(f)
     return f
 
 
@@ -159,12 +167,13 @@ def copy_current_request_context(f: t.Callable) -> t.Callable:
     .. versionadded:: 0.10
     """
     top = _request_ctx_stack.top
+
     if top is None:
         raise RuntimeError(
-            "This decorator can only be used at local scopes "
-            "when a request context is on the stack.  For instance within "
-            "view functions."
+            "This decorator can only be used when a request context is"
+            " active, such as within a view function."
         )
+
     reqctx = top.copy()
 
     def wrapper(*args, **kwargs):
diff --git a/src/flask/json/__init__.py b/src/flask/json/__init__.py
index eac2f576..43c018e7 100644
--- a/src/flask/json/__init__.py
+++ b/src/flask/json/__init__.py
@@ -74,6 +74,11 @@ def _dump_arg_defaults(
         if bp is not None and bp.json_encoder is not None:
             cls = bp.json_encoder
 
+        # Only set a custom encoder if it has custom behavior. This is
+        # faster on PyPy.
+        if cls is not _json.JSONEncoder:
+            kwargs.setdefault("cls", cls)
+
         kwargs.setdefault("cls", cls)
         kwargs.setdefault("ensure_ascii", app.config["JSON_AS_ASCII"])
         kwargs.setdefault("sort_keys", app.config["JSON_SORT_KEYS"])
@@ -95,9 +100,10 @@ def _load_arg_defaults(
         if bp is not None and bp.json_decoder is not None:
             cls = bp.json_decoder
 
-        kwargs.setdefault("cls", cls)
-    else:
-        kwargs.setdefault("cls", JSONDecoder)
+        # Only set a custom decoder if it has custom behavior. This is
+        # faster on PyPy.
+        if cls not in {JSONDecoder, _json.JSONDecoder}:
+            kwargs.setdefault("cls", cls)
 
 
 def dumps(obj: t.Any, app: t.Optional["Flask"] = None, **kwargs: t.Any) -> str: