Don't get full channel on iter_participants unless necessary

telethon/telegram_client.py

@@ -1247,11 +1247,14 @@ class TelegramClient(TelegramBareClient):
 
         limit = float('inf') if limit is None else int(limit)
         if isinstance(entity, InputPeerChannel):
+            if _total or (aggressive and not filter):
                 total = self(GetFullChannelRequest(
                     entity
                 )).full_chat.participants_count
                 if _total:
                     _total[0] = total
+            else:
+                total = 0
 
             if limit == 0:
                 return