diff --git a/httpcore/_async/connection_pool.py b/httpcore/_async/connection_pool.py
index 214dfc4be..440858cb8 100644
--- a/httpcore/_async/connection_pool.py
+++ b/httpcore/_async/connection_pool.py
@@ -1,7 +1,14 @@
 import ssl
 import sys
 from types import TracebackType
-from typing import AsyncIterable, AsyncIterator, Iterable, List, Optional, Type
+from typing import (
+    AsyncIterable,
+    AsyncIterator,
+    Iterable,
+    List,
+    Optional,
+    Type,
+)
 
 from .._backends.auto import AutoBackend
 from .._backends.base import SOCKET_OPTION, AsyncNetworkBackend
@@ -238,6 +245,7 @@ def _assign_requests_to_connections(self) -> List[AsyncConnectionInterface]:
         those connections to be handled seperately.
         """
         closing_connections = []
+        idling_count = 0
 
         # First we handle cleaning up any connections that are closed,
         # have expired their keep-alive, or surplus idle connections.
@@ -249,27 +257,29 @@ def _assign_requests_to_connections(self) -> List[AsyncConnectionInterface]:
                 # log: "closing expired connection"
                 self._connections.remove(connection)
                 closing_connections.append(connection)
-            elif (
-                connection.is_idle()
-                and len([connection.is_idle() for connection in self._connections])
-                > self._max_keepalive_connections
-            ):
+            elif connection.is_idle():
+                if idling_count < self._max_keepalive_connections:
+                    idling_count += 1
+                    continue
                 # log: "closing idle connection"
                 self._connections.remove(connection)
                 closing_connections.append(connection)
 
         # Assign queued requests to connections.
-        queued_requests = [request for request in self._requests if request.is_queued()]
-        for pool_request in queued_requests:
+        for pool_request in list(self._requests):
+            if not pool_request.is_queued():
+                continue
+
             origin = pool_request.request.url.origin
-            available_connections = [
-                connection
-                for connection in self._connections
-                if connection.can_handle_request(origin) and connection.is_available()
-            ]
-            idle_connections = [
-                connection for connection in self._connections if connection.is_idle()
-            ]
+            available_connection = next(
+                (
+                    connection
+                    for connection in self._connections
+                    if connection.can_handle_request(origin)
+                    and connection.is_available()
+                ),
+                None,
+            )
 
             # There are three cases for how we may be able to handle the request:
             #
@@ -277,24 +287,26 @@ def _assign_requests_to_connections(self) -> List[AsyncConnectionInterface]:
             # 2. We can create a new connection to handle the request.
             # 3. We can close an idle connection and then create a new connection
             #    to handle the request.
-            if available_connections:
+            if available_connection is not None:
                 # log: "reusing existing connection"
-                connection = available_connections[0]
-                pool_request.assign_to_connection(connection)
+                pool_request.assign_to_connection(available_connection)
             elif len(self._connections) < self._max_connections:
                 # log: "creating new connection"
                 connection = self.create_connection(origin)
                 self._connections.append(connection)
                 pool_request.assign_to_connection(connection)
-            elif idle_connections:
-                # log: "closing idle connection"
-                connection = idle_connections[0]
-                self._connections.remove(connection)
-                closing_connections.append(connection)
-                # log: "creating new connection"
-                connection = self.create_connection(origin)
-                self._connections.append(connection)
-                pool_request.assign_to_connection(connection)
+            else:
+                idling_connection = next(
+                    (c for c in self._connections if c.is_idle()), None
+                )
+                if idling_connection is not None:
+                    # log: "closing idle connection"
+                    self._connections.remove(idling_connection)
+                    closing_connections.append(idling_connection)
+                    # log: "creating new connection"
+                    new_connection = self.create_connection(origin)
+                    self._connections.append(new_connection)
+                    pool_request.assign_to_connection(new_connection)
 
         return closing_connections
 
diff --git a/httpcore/_sync/connection_pool.py b/httpcore/_sync/connection_pool.py
index 01bec59e8..61961eef9 100644
--- a/httpcore/_sync/connection_pool.py
+++ b/httpcore/_sync/connection_pool.py
@@ -1,7 +1,14 @@
 import ssl
 import sys
 from types import TracebackType
-from typing import Iterable, Iterator, Iterable, List, Optional, Type
+from typing import (
+    Iterable,
+    Iterator,
+    Iterable,
+    List,
+    Optional,
+    Type,
+)
 
 from .._backends.sync import SyncBackend
 from .._backends.base import SOCKET_OPTION, NetworkBackend
@@ -238,6 +245,7 @@ def _assign_requests_to_connections(self) -> List[ConnectionInterface]:
         those connections to be handled seperately.
         """
         closing_connections = []
+        idling_count = 0
 
         # First we handle cleaning up any connections that are closed,
         # have expired their keep-alive, or surplus idle connections.
@@ -249,27 +257,29 @@ def _assign_requests_to_connections(self) -> List[ConnectionInterface]:
                 # log: "closing expired connection"
                 self._connections.remove(connection)
                 closing_connections.append(connection)
-            elif (
-                connection.is_idle()
-                and len([connection.is_idle() for connection in self._connections])
-                > self._max_keepalive_connections
-            ):
+            elif connection.is_idle():
+                if idling_count < self._max_keepalive_connections:
+                    idling_count += 1
+                    continue
                 # log: "closing idle connection"
                 self._connections.remove(connection)
                 closing_connections.append(connection)
 
         # Assign queued requests to connections.
-        queued_requests = [request for request in self._requests if request.is_queued()]
-        for pool_request in queued_requests:
+        for pool_request in list(self._requests):
+            if not pool_request.is_queued():
+                continue
+
             origin = pool_request.request.url.origin
-            available_connections = [
-                connection
-                for connection in self._connections
-                if connection.can_handle_request(origin) and connection.is_available()
-            ]
-            idle_connections = [
-                connection for connection in self._connections if connection.is_idle()
-            ]
+            available_connection = next(
+                (
+                    connection
+                    for connection in self._connections
+                    if connection.can_handle_request(origin)
+                    and connection.is_available()
+                ),
+                None,
+            )
 
             # There are three cases for how we may be able to handle the request:
             #
@@ -277,24 +287,26 @@ def _assign_requests_to_connections(self) -> List[ConnectionInterface]:
             # 2. We can create a new connection to handle the request.
             # 3. We can close an idle connection and then create a new connection
             #    to handle the request.
-            if available_connections:
+            if available_connection is not None:
                 # log: "reusing existing connection"
-                connection = available_connections[0]
-                pool_request.assign_to_connection(connection)
+                pool_request.assign_to_connection(available_connection)
             elif len(self._connections) < self._max_connections:
                 # log: "creating new connection"
                 connection = self.create_connection(origin)
                 self._connections.append(connection)
                 pool_request.assign_to_connection(connection)
-            elif idle_connections:
-                # log: "closing idle connection"
-                connection = idle_connections[0]
-                self._connections.remove(connection)
-                closing_connections.append(connection)
-                # log: "creating new connection"
-                connection = self.create_connection(origin)
-                self._connections.append(connection)
-                pool_request.assign_to_connection(connection)
+            else:
+                idling_connection = next(
+                    (c for c in self._connections if c.is_idle()), None
+                )
+                if idling_connection is not None:
+                    # log: "closing idle connection"
+                    self._connections.remove(idling_connection)
+                    closing_connections.append(idling_connection)
+                    # log: "creating new connection"
+                    new_connection = self.create_connection(origin)
+                    self._connections.append(new_connection)
+                    pool_request.assign_to_connection(new_connection)
 
         return closing_connections