diff --git a/whisper/__init__.py b/whisper/__init__.py
index e210718f3..bfe0a8984 100644
--- a/whisper/__init__.py
+++ b/whisper/__init__.py
@@ -60,11 +60,20 @@ def _download(url: str, root: str, in_memory: bool) -> Union[bytes, str]:
     if os.path.exists(download_target) and not os.path.isfile(download_target):
         raise RuntimeError(f"{download_target} exists and is not a regular file")
 
+    def compute_sha256(file_path: str) -> str:
+        sha256 = hashlib.sha256()
+        with open(file_path, "rb") as f:
+            for chunk in iter(lambda: f.read(8192), b""):
+                sha256.update(chunk)
+        return sha256.hexdigest()
+
     if os.path.isfile(download_target):
-        with open(download_target, "rb") as f:
-            model_bytes = f.read()
-        if hashlib.sha256(model_bytes).hexdigest() == expected_sha256:
-            return model_bytes if in_memory else download_target
+        if compute_sha256(download_target) == expected_sha256:
+            if in_memory:
+                with open(download_target, "rb") as f:
+                    return f.read()
+            else:
+                return download_target
         else:
             warnings.warn(
                 f"{download_target} exists, but the SHA256 checksum does not match; re-downloading the file"
@@ -86,13 +95,16 @@ def _download(url: str, root: str, in_memory: bool) -> Union[bytes, str]:
                 output.write(buffer)
                 loop.update(len(buffer))
 
-    model_bytes = open(download_target, "rb").read()
-    if hashlib.sha256(model_bytes).hexdigest() != expected_sha256:
+    if compute_sha256(download_target) != expected_sha256:
         raise RuntimeError(
             "Model has been downloaded but the SHA256 checksum does not not match. Please retry loading the model."
         )
 
-    return model_bytes if in_memory else download_target
+    if in_memory:
+        with open(download_target, "rb") as f:
+            return f.read()
+    else:
+        return download_target
 
 
 def available_models() -> List[str]:
@@ -157,4 +169,4 @@ def load_model(
     if alignment_heads is not None:
         model.set_alignment_heads(alignment_heads)
 
-    return model.to(device)
+    return model.to(device)
\ No newline at end of file
diff --git a/whisper/model.py b/whisper/model.py
index e53744738..072f0a4f5 100644
--- a/whisper/model.py
+++ b/whisper/model.py
@@ -224,31 +224,64 @@ def __init__(
         mask = torch.empty(n_ctx, n_ctx).fill_(-np.inf).triu_(1)
         self.register_buffer("mask", mask, persistent=False)
 
-    def forward(self, x: Tensor, xa: Tensor, kv_cache: Optional[dict] = None):
+        # Optimisation: pre-compute and register the mask in CUDA if available
+        if torch.cuda.is_available():
+            self.register_buffer("mask_cuda", mask.cuda(), persistent=False)
+
+
+    def forward(self, tokens: Tensor, audio_features: Tensor, kv_cache: Optional[dict] = None) -> Tensor:
         """
-        x : torch.LongTensor, shape = (batch_size, <= n_ctx)
-            the text tokens
-        xa : torch.Tensor, shape = (batch_size, n_audio_ctx, n_audio_state)
-            the encoded audio features to be attended on
+        Args:
+            tokens: (n_batch, n_token)
+            audio_features: (n_batch, n_audio_ctx, n_audio_state)
+            kv_cache: Optional cache for key/value tensors
+
+        Returns:
+            logits: (n_batch, n_token, n_vocab)
         """
-        offset = next(iter(kv_cache.values())).shape[1] if kv_cache else 0
-        x = (
-            self.token_embedding(x)
-            + self.positional_embedding[offset : offset + x.shape[-1]]
-        )
-        x = x.to(xa.dtype)
+        n_batch, n_token = tokens.shape
+        
+        # Get the dtype of audio_features to ensure consistency
+        dtype = audio_features.dtype
+        
+        # Handle kv_cache for token embedding offset
+        if kv_cache is not None:
+            offset = next(iter(kv_cache.values())).shape[1] if kv_cache else 0
+            x = self.token_embedding(tokens) + self.positional_embedding[offset:offset + tokens.shape[1]]
+        else:
+            x = self.token_embedding(tokens) + self.positional_embedding[:n_token]
+        
+        # Convert to the same dtype as audio_features
+        x = x.to(dtype)
 
+        # Optimisation: Move audio_features to GPU once here.
+        if torch.cuda.is_available():
+            audio_features = audio_features.cuda()
+
+        # Process through attention blocks
         for block in self.blocks:
-            x = block(x, xa, mask=self.mask, kv_cache=kv_cache)
+            x = block(x, audio_features, kv_cache=kv_cache)
 
         x = self.ln(x)
-        logits = (
-            x @ torch.transpose(self.token_embedding.weight.to(x.dtype), 0, 1)
-        ).float()
+        
+        # Ensure consistent dtype for matrix multiplication
+        # Convert token_embedding weight to the same dtype as x
+        embedding_weights = self.token_embedding.weight.to(x.dtype)
+        logits = x @ embedding_weights.T
+
+        # Apply mask if not using kv_cache (inference)
+        if kv_cache is None:
+            # Optimisation: Apply the precomputed CUDA mask if available.
+            if torch.cuda.is_available():
+                mask = self.mask_cuda[:n_token, :n_token]
+            else:
+                mask = self.mask[:n_token, :n_token]
+            
+            logits = logits + mask
 
         return logits
-
-
+    
+# The Whisper class has been moved outside of TextDecoder and is now a top-level class
 class Whisper(nn.Module):
     def __init__(self, dims: ModelDimensions):
         super().__init__()