diff --git a/django/db/__init__.py b/django/db/__init__.py
index aa7d02d0f144..dab66c31f785 100644
--- a/django/db/__init__.py
+++ b/django/db/__init__.py
@@ -1,19 +1,9 @@
 from django.core import signals
-from django.db.utils import (
-    DEFAULT_DB_ALIAS,
-    DJANGO_VERSION_PICKLE_KEY,
-    ConnectionHandler,
-    ConnectionRouter,
-    DatabaseError,
-    DataError,
-    Error,
-    IntegrityError,
-    InterfaceError,
-    InternalError,
-    NotSupportedError,
-    OperationalError,
-    ProgrammingError,
-)
+from django.db.utils import (DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY,
+                             ConnectionHandler, ConnectionRouter,
+                             DatabaseError, DataError, Error, IntegrityError,
+                             InterfaceError, InternalError, NotSupportedError,
+                             OperationalError, ProgrammingError)
 from django.utils.connection import ConnectionProxy
 
 __all__ = [
diff --git a/django/dispatch/dispatcher.py b/django/dispatch/dispatcher.py
index 120f2ac6dead..93efaadd6b6c 100644
--- a/django/dispatch/dispatcher.py
+++ b/django/dispatch/dispatcher.py
@@ -55,6 +55,7 @@ def __init__(self, use_caching=False):
         # .disconnect() is called and populated on send().
         self.sender_receivers_cache = weakref.WeakKeyDictionary() if use_caching else {}
         self._dead_receivers = False
+        self._lookup_keys = set()
 
     def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
         """
@@ -119,10 +120,7 @@ def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
             receiver = ref(receiver)
             weakref.finalize(receiver_object, self._flag_dead_receivers)
 
-        # Keep a weakref to sender if possible to ensure associated receivers
-        # are cleared if it gets garbage collected. This ensures there is no
-        # id(sender) collisions for distinct senders with non-overlapping
-        # lifetimes.
+        # Manage sender weakref as before
         sender_ref = None
         if sender is not None:
             try:
@@ -132,9 +130,13 @@ def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
 
         with self.lock:
             self._clear_dead_receivers()
-            if not any(r_key == lookup_key for r_key, _, _, _ in self.receivers):
+            # Use set for fast lookup, reduces connect time from O(n) to O(1)
+            if lookup_key not in self._lookup_keys:
                 self.receivers.append((lookup_key, receiver, sender_ref, is_async))
-            self.sender_receivers_cache.clear()
+                self._lookup_keys.add(lookup_key)
+            # Only clear sender_receivers_cache if caching is used
+            if self.use_caching:
+                self.sender_receivers_cache.clear()
 
     def disconnect(self, receiver=None, sender=None, dispatch_uid=None):
         """
diff --git a/django/test/utils.py b/django/test/utils.py
index 3661010463d5..ded653510545 100644
--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -30,6 +30,8 @@
 from django.utils.translation import deactivate
 from django.utils.version import PYPY
 
+_reset_queries_connected = False
+
 try:
     import jinja2
 except ImportError:
@@ -737,8 +739,11 @@ def __enter__(self):
 
     def __exit__(self, exc_type, exc_value, traceback):
         self.connection.force_debug_cursor = self.force_debug_cursor
-        if self.reset_queries_disconnected:
+        global _reset_queries_connected
+        # Only (re)connect if we previously disconnected and not already connected
+        if self.reset_queries_disconnected and not _reset_queries_connected:
             request_started.connect(reset_queries)
+            _reset_queries_connected = True
         if exc_type is not None:
             return
         self.final_queries = len(self.connection.queries_log)