diff --git a/blanc/utils.py b/blanc/utils.py
index fa97402..d261b5e 100644
--- a/blanc/utils.py
+++ b/blanc/utils.py
@@ -222,25 +222,29 @@ def mask_tokens_randomly(tokens, min_token_lengths, mask_token, p_mask):
     """
     n_mask = max(int(len(tokens) * p_mask), 1)
 
-    token_positions = []
-    for idx, token in enumerate(tokens):
-        next_token = '' if idx + 1 == len(tokens) else tokens[idx + 1]
-        if is_token_large_enough(token, next_token, min_token_lengths):
-            token_positions.append(idx)
+    # Precompute next_token list once for efficiency
+    next_tokens = tokens[1:] + ['']
+    token_positions = [
+        idx for idx, (token, next_token) in enumerate(zip(tokens, next_tokens))
+        if is_token_large_enough(token, next_token, min_token_lengths)
+    ]
+
     random.shuffle(token_positions)
 
-    all_inputs, all_answers = [], []
-    while len(token_positions) > 0:
-        positions_to_mask = token_positions[:n_mask]
-        token_positions = token_positions[n_mask:]
+    # Avoid repeated lookup of positions_to_mask
+    all_inputs = []
+    all_answers = []
+    position_count = len(token_positions)
+    if position_count == 0:
+        return all_inputs, all_answers
 
-        inputs, answers = [], {}
-        for idx, token in enumerate(tokens):
-            if idx in positions_to_mask:
-                inputs.append(mask_token)
-                answers[idx] = token
-            else:
-                inputs.append(token)
+    # Make a copy to avoid resizing list in every iteration (efficient slicing)
+    for start in range(0, position_count, n_mask):
+        positions_to_mask = set(token_positions[start:start + n_mask])
+
+        # Use list comprehension for inputs; dict comprehension for answers
+        inputs = [mask_token if idx in positions_to_mask else token for idx, token in enumerate(tokens)]
+        answers = {idx: tokens[idx] for idx in positions_to_mask}
 
         all_inputs.append(inputs)
         all_answers.append(answers)