diff --git a/homeassistant/components/config/device_registry.py b/homeassistant/components/config/device_registry.py
index dfa55b02c30..aa861a515fd 100644
--- a/homeassistant/components/config/device_registry.py
+++ b/homeassistant/components/config/device_registry.py
@@ -47,15 +47,14 @@ def websocket_list_devices(
         f'"success":true,"result": ['
     ).encode()
     # Concatenate cached entity registry item JSON serializations
-    msg_json = (
-        msg_json_prefix
-        + b",".join(
+    inner = b",".join(
+        [
             entry.json_repr
             for entry in registry.devices.values()
             if entry.json_repr is not None
-        )
-        + b"]}"
+        ]
     )
+    msg_json = b"".join((msg_json_prefix, inner, b"]}"))
     connection.send_message(msg_json)
 
 
diff --git a/homeassistant/components/config/entity_registry.py b/homeassistant/components/config/entity_registry.py
index f1c1fadc144..11a4617adfa 100644
--- a/homeassistant/components/config/entity_registry.py
+++ b/homeassistant/components/config/entity_registry.py
@@ -45,15 +45,14 @@ def websocket_list_entities(
         '"success":true,"result": ['
     ).encode()
     # Concatenate cached entity registry item JSON serializations
-    msg_json = (
-        msg_json_prefix
-        + b",".join(
+    inner = b",".join(
+        [
             entry.partial_json_repr
             for entry in registry.entities.values()
             if entry.partial_json_repr is not None
-        )
-        + b"]}"
+        ]
     )
+    msg_json = b"".join((msg_json_prefix, inner, b"]}"))
     connection.send_message(msg_json)
 
 
@@ -77,15 +76,14 @@ def websocket_list_entities_for_display(
         f'"result":{{"entity_categories":{_ENTITY_CATEGORIES_JSON},"entities":['
     ).encode()
     # Concatenate cached entity registry item JSON serializations
-    msg_json = (
-        msg_json_prefix
-        + b",".join(
+    inner = b",".join(
+        [
             entry.display_json_repr
             for entry in registry.entities.values()
             if entry.disabled_by is None and entry.display_json_repr is not None
-        )
-        + b"]}}"
+        ]
     )
+    msg_json = b"".join((msg_json_prefix, inner, b"]}}"))
     connection.send_message(msg_json)