diff --git a/src/transformers/models/prophetnet/tokenization_prophetnet.py b/src/transformers/models/prophetnet/tokenization_prophetnet.py
index 24401835c7fc..efac7145d675 100644
--- a/src/transformers/models/prophetnet/tokenization_prophetnet.py
+++ b/src/transformers/models/prophetnet/tokenization_prophetnet.py
@@ -354,6 +354,11 @@ def __init__(
             )
         self.wordpiece_tokenizer = WordpieceTokenizer(vocab=self.vocab, unk_token=str(unk_token))
 
+        # Cache unknown token id for use in _convert_token_to_id
+        self._unk_token_id = self.vocab.get(
+            self.unk_token if hasattr(self, "unk_token") and self.unk_token is not None else unk_token
+        )
+
         super().__init__(
             do_lower_case=do_lower_case,
             do_basic_tokenize=do_basic_tokenize,
@@ -391,7 +396,11 @@ def _tokenize(self, text):
 
     def _convert_token_to_id(self, token: str):
         """Converts a token (str) in an id using the vocab."""
-        return self.vocab.get(token, self.vocab.get(self.unk_token))
+        # Optimize the fallback by caching unk id in __init__ to avoid repeated dictionary lookups
+        id_ = self.vocab.get(token)
+        if id_ is not None:
+            return id_
+        return self._unk_token_id
 
     def _convert_id_to_token(self, index: int):
         """Converts an index (integer) in a token (str) using the vocab."""