diff --git a/modules/misc.py b/modules/misc.py
index 724cda7..753af9f 100644
--- a/modules/misc.py
+++ b/modules/misc.py
@@ -15,15 +15,22 @@ class ProcessedPostsManager:
             return set()
 
     def _save(self):
+        """Full rewrite — use for batch operations only."""
         with open(self.file_path, "w", encoding="utf-8") as processed:
-            processed.write("\n".join(self.processed_posts))
+            processed.write("".join(f"{pid}\n" for pid in self.processed_posts))
 
     def is_processed(self, post_id: int) -> bool:
         return str(post_id) in self.processed_posts
 
     def mark_processed(self, post_id: int):
-        self.processed_posts.add(str(post_id))
-        self._save()
+        """Mark a single post processed and append to file (no full rewrite)."""
+        str_id = str(post_id)
+        if str_id in self.processed_posts:
+            return
+
+        self.processed_posts.add(str_id)
+        with open(self.file_path, "a", encoding="utf-8") as f:
+            f.write(str_id + "\n")
 
     def add_existing_posts(self, post_ids: List[int]):
         for post_id in post_ids: