diff --git a/telethon/client/chats.py b/telethon/client/chats.py
index 1d42b40e..08242f0e 100644
--- a/telethon/client/chats.py
+++ b/telethon/client/chats.py
@@ -877,8 +877,12 @@ class ChatMethods:
             # would get a RIGHT_FORBIDDEN. However, it makes sense
             # that an admin can post messages, so we want to avoid the error
             if post_messages or edit_messages:
-                full_entity = await self.get_entity(entity)
-                if full_entity.megagroup:
+                # TODO get rid of this once sessions cache this information
+                if entity.channel_id not in self._megagroup_cache:
+                    full_entity = await self.get_entity(entity)
+                    self._megagroup_cache[entity.channel_id] = full_entity.megagroup
+
+                if self._megagroup_cache[entity.channel_id]:
                     post_messages = None
                     edit_messages = None
 
diff --git a/telethon/client/telegrambaseclient.py b/telethon/client/telegrambaseclient.py
index b0882e6c..26f17a92 100644
--- a/telethon/client/telegrambaseclient.py
+++ b/telethon/client/telegrambaseclient.py
@@ -340,6 +340,9 @@ class TelegramBaseClient(abc.ABC):
         self._self_input_peer = None
         self._bot = None
 
+        # A place to store if channels are a megagroup or not (see `edit_admin`)
+        self._megagroup_cache = {}
+
     # endregion
 
     # region Properties