diff --git a/doc/whats_new/v1.0.rst b/doc/whats_new/v1.0.rst
index 40076c0b275dd..21c150e49de37 100644
--- a/doc/whats_new/v1.0.rst
+++ b/doc/whats_new/v1.0.rst
@@ -43,6 +43,14 @@ Fixed models
   between sparse and dense input. :pr:`21195`
   by :user:`Jérémie du Boisberranger <jeremiedbb>`.
 
+:mod:`sklearn.feature_extraction`
+.................................
+
+- |Efficiency| Fixed an efficiency regression introduced in version 1.0.0 in the
+  `transform` method of :class:`feature_extraction.text.CountVectorizer` which no
+  longer checks for uppercase characters in the provided vocabulary. :pr:`21251`
+  by :user:`Jérémie du Boisberranger <jeremiedbb>`.
+
 :mod:`sklearn.linear_model`
 ...........................
 
diff --git a/sklearn/feature_extraction/tests/test_text.py b/sklearn/feature_extraction/tests/test_text.py
index 6abd731b4559a..da32e855fabb6 100644
--- a/sklearn/feature_extraction/tests/test_text.py
+++ b/sklearn/feature_extraction/tests/test_text.py
@@ -436,7 +436,9 @@ def test_countvectorizer_custom_token_pattern_with_several_group():
 
 
 def test_countvectorizer_uppercase_in_vocab():
-    vocabulary = ["Sample", "Upper", "CaseVocabulary"]
+    # Check that the check for uppercase in the provided vocabulary is only done at fit
+    # time and not at transform time (#21251)
+    vocabulary = ["Sample", "Upper", "Case", "Vocabulary"]
     message = (
         "Upper case characters found in"
         " vocabulary while 'lowercase'"
@@ -445,8 +447,13 @@ def test_countvectorizer_uppercase_in_vocab():
     )
 
     vectorizer = CountVectorizer(lowercase=True, vocabulary=vocabulary)
+
     with pytest.warns(UserWarning, match=message):
-        vectorizer.fit_transform(vocabulary)
+        vectorizer.fit(vocabulary)
+
+    with pytest.warns(None) as record:
+        vectorizer.transform(vocabulary)
+    assert not record
 
 
 def test_tf_transformer_feature_names_out():
diff --git a/sklearn/feature_extraction/text.py b/sklearn/feature_extraction/text.py
index a64bbc3ff3737..9124edb455b71 100644
--- a/sklearn/feature_extraction/text.py
+++ b/sklearn/feature_extraction/text.py
@@ -1194,17 +1194,6 @@ def _count_vocab(self, raw_documents, fixed_vocab):
         j_indices = []
         indptr = []
 
-        if self.lowercase:
-            for vocab in vocabulary:
-                if any(map(str.isupper, vocab)):
-                    warnings.warn(
-                        "Upper case characters found in"
-                        " vocabulary while 'lowercase'"
-                        " is True. These entries will not"
-                        " be matched with any documents"
-                    )
-                    break
-
         values = _make_int_array()
         indptr.append(0)
         for doc in raw_documents:
@@ -1327,6 +1316,17 @@ def fit_transform(self, raw_documents, y=None):
         min_df = self.min_df
         max_features = self.max_features
 
+        if self.fixed_vocabulary_ and self.lowercase:
+            for term in self.vocabulary:
+                if any(map(str.isupper, term)):
+                    warnings.warn(
+                        "Upper case characters found in"
+                        " vocabulary while 'lowercase'"
+                        " is True. These entries will not"
+                        " be matched with any documents"
+                    )
+                    break
+
         vocabulary, X = self._count_vocab(raw_documents, self.fixed_vocabulary_)
 
         if self.binary: