diff --git a/changes/3721.misc.md b/changes/3721.misc.md
new file mode 100644
index 0000000000..c170712882
--- /dev/null
+++ b/changes/3721.misc.md
@@ -0,0 +1 @@
+Adds synchronous (non-async) encoding and decoding methods to CPU-bound codecs. This is necessary for performance optimizations based on avoiding `asyncio` overhead. These new methods are described by a new protocol: `SupportsSyncCodec`.
\ No newline at end of file
diff --git a/src/zarr/abc/codec.py b/src/zarr/abc/codec.py
index d41c457b4e..3ec5ec522b 100644
--- a/src/zarr/abc/codec.py
+++ b/src/zarr/abc/codec.py
@@ -2,7 +2,7 @@
 
 from abc import abstractmethod
 from collections.abc import Mapping
-from typing import TYPE_CHECKING, Generic, TypeGuard, TypeVar
+from typing import TYPE_CHECKING, Generic, Protocol, TypeGuard, TypeVar, runtime_checkable
 
 from typing_extensions import ReadOnly, TypedDict
 
@@ -32,6 +32,7 @@
     "CodecInput",
     "CodecOutput",
     "CodecPipeline",
+    "SupportsSyncCodec",
 ]
 
 CodecInput = TypeVar("CodecInput", bound=NDBuffer | Buffer)
@@ -59,6 +60,23 @@ def _check_codecjson_v2(data: object) -> TypeGuard[CodecJSON_V2[str]]:
 """The widest type of JSON-like input that could specify a codec."""
 
 
+@runtime_checkable
+class SupportsSyncCodec(Protocol):
+    """Protocol for codecs that support synchronous encode/decode.
+
+    Codecs implementing this protocol provide ``_decode_sync`` and ``_encode_sync``
+    methods that perform encoding/decoding without requiring an async event loop.
+    """
+
+    def _decode_sync(
+        self, chunk_data: NDBuffer | Buffer, chunk_spec: ArraySpec
+    ) -> NDBuffer | Buffer: ...
+
+    def _encode_sync(
+        self, chunk_data: NDBuffer | Buffer, chunk_spec: ArraySpec
+    ) -> NDBuffer | Buffer | None: ...
+
+
 class BaseCodec(Metadata, Generic[CodecInput, CodecOutput]):
     """Generic base class for codecs.
 
diff --git a/src/zarr/codecs/blosc.py b/src/zarr/codecs/blosc.py
index 5b91cfa005..62ceff7659 100644
--- a/src/zarr/codecs/blosc.py
+++ b/src/zarr/codecs/blosc.py
@@ -299,28 +299,37 @@ def _blosc_codec(self) -> Blosc:
             config_dict["typesize"] = self.typesize
         return Blosc.from_config(config_dict)
 
+    def _decode_sync(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer:
+        return as_numpy_array_wrapper(self._blosc_codec.decode, chunk_bytes, chunk_spec.prototype)
+
     async def _decode_single(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await asyncio.to_thread(
-            as_numpy_array_wrapper, self._blosc_codec.decode, chunk_bytes, chunk_spec.prototype
-        )
+        return await asyncio.to_thread(self._decode_sync, chunk_bytes, chunk_spec)
 
-    async def _encode_single(
+    def _encode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer | None:
         # Since blosc only support host memory, we convert the input and output of the encoding
         # between numpy array and buffer
-        return await asyncio.to_thread(
-            lambda chunk: chunk_spec.prototype.buffer.from_bytes(
-                self._blosc_codec.encode(chunk.as_numpy_array())
-            ),
-            chunk_bytes,
+        return chunk_spec.prototype.buffer.from_bytes(
+            self._blosc_codec.encode(chunk_bytes.as_numpy_array())
         )
 
+    async def _encode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return await asyncio.to_thread(self._encode_sync, chunk_bytes, chunk_spec)
+
     def compute_encoded_size(self, _input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         raise NotImplementedError
diff --git a/src/zarr/codecs/bytes.py b/src/zarr/codecs/bytes.py
index 1fbdeef497..86bb354fb5 100644
--- a/src/zarr/codecs/bytes.py
+++ b/src/zarr/codecs/bytes.py
@@ -65,7 +65,7 @@ def evolve_from_array_spec(self, array_spec: ArraySpec) -> Self:
             )
         return self
 
-    async def _decode_single(
+    def _decode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
@@ -88,7 +88,14 @@ async def _decode_single(
             )
         return chunk_array
 
-    async def _encode_single(
+    async def _decode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> NDBuffer:
+        return self._decode_sync(chunk_bytes, chunk_spec)
+
+    def _encode_sync(
         self,
         chunk_array: NDBuffer,
         chunk_spec: ArraySpec,
@@ -109,5 +116,12 @@ async def _encode_single(
         nd_array = nd_array.ravel().view(dtype="B")
         return chunk_spec.prototype.buffer.from_array_like(nd_array)
 
+    async def _encode_single(
+        self,
+        chunk_array: NDBuffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return self._encode_sync(chunk_array, chunk_spec)
+
     def compute_encoded_size(self, input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         return input_byte_length
diff --git a/src/zarr/codecs/crc32c_.py b/src/zarr/codecs/crc32c_.py
index 9536d0d558..ebe2ac8f7a 100644
--- a/src/zarr/codecs/crc32c_.py
+++ b/src/zarr/codecs/crc32c_.py
@@ -31,7 +31,7 @@ def from_dict(cls, data: dict[str, JSON]) -> Self:
     def to_dict(self) -> dict[str, JSON]:
         return {"name": "crc32c"}
 
-    async def _decode_single(
+    def _decode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
@@ -51,7 +51,14 @@ async def _decode_single(
             )
         return chunk_spec.prototype.buffer.from_array_like(inner_bytes)
 
-    async def _encode_single(
+    async def _decode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer:
+        return self._decode_sync(chunk_bytes, chunk_spec)
+
+    def _encode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
@@ -64,5 +71,12 @@ async def _encode_single(
         # Append the checksum (as bytes) to the data
         return chunk_spec.prototype.buffer.from_array_like(np.append(data, checksum.view("B")))
 
+    async def _encode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return self._encode_sync(chunk_bytes, chunk_spec)
+
     def compute_encoded_size(self, input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         return input_byte_length + 4
diff --git a/src/zarr/codecs/gzip.py b/src/zarr/codecs/gzip.py
index 610ca9dadd..b8591748f7 100644
--- a/src/zarr/codecs/gzip.py
+++ b/src/zarr/codecs/gzip.py
@@ -2,6 +2,7 @@
 
 import asyncio
 from dataclasses import dataclass
+from functools import cached_property
 from typing import TYPE_CHECKING
 
 from numcodecs.gzip import GZip
@@ -48,23 +49,37 @@ def from_dict(cls, data: dict[str, JSON]) -> Self:
     def to_dict(self) -> dict[str, JSON]:
         return {"name": "gzip", "configuration": {"level": self.level}}
 
+    @cached_property
+    def _gzip_codec(self) -> GZip:
+        return GZip(self.level)
+
+    def _decode_sync(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer:
+        return as_numpy_array_wrapper(self._gzip_codec.decode, chunk_bytes, chunk_spec.prototype)
+
     async def _decode_single(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await asyncio.to_thread(
-            as_numpy_array_wrapper, GZip(self.level).decode, chunk_bytes, chunk_spec.prototype
-        )
+        return await asyncio.to_thread(self._decode_sync, chunk_bytes, chunk_spec)
+
+    def _encode_sync(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return as_numpy_array_wrapper(self._gzip_codec.encode, chunk_bytes, chunk_spec.prototype)
 
     async def _encode_single(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer | None:
-        return await asyncio.to_thread(
-            as_numpy_array_wrapper, GZip(self.level).encode, chunk_bytes, chunk_spec.prototype
-        )
+        return await asyncio.to_thread(self._encode_sync, chunk_bytes, chunk_spec)
 
     def compute_encoded_size(
         self,
diff --git a/src/zarr/codecs/transpose.py b/src/zarr/codecs/transpose.py
index a8570b6e8f..609448a59c 100644
--- a/src/zarr/codecs/transpose.py
+++ b/src/zarr/codecs/transpose.py
@@ -95,20 +95,34 @@ def resolve_metadata(self, chunk_spec: ArraySpec) -> ArraySpec:
             prototype=chunk_spec.prototype,
         )
 
-    async def _decode_single(
+    def _decode_sync(
         self,
         chunk_array: NDBuffer,
         chunk_spec: ArraySpec,
     ) -> NDBuffer:
-        inverse_order = np.argsort(self.order)
+        inverse_order = tuple(int(i) for i in np.argsort(self.order))
         return chunk_array.transpose(inverse_order)
 
-    async def _encode_single(
+    async def _decode_single(
+        self,
+        chunk_array: NDBuffer,
+        chunk_spec: ArraySpec,
+    ) -> NDBuffer:
+        return self._decode_sync(chunk_array, chunk_spec)
+
+    def _encode_sync(
         self,
         chunk_array: NDBuffer,
         _chunk_spec: ArraySpec,
     ) -> NDBuffer | None:
         return chunk_array.transpose(self.order)
 
+    async def _encode_single(
+        self,
+        chunk_array: NDBuffer,
+        _chunk_spec: ArraySpec,
+    ) -> NDBuffer | None:
+        return self._encode_sync(chunk_array, _chunk_spec)
+
     def compute_encoded_size(self, input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         return input_byte_length
diff --git a/src/zarr/codecs/vlen_utf8.py b/src/zarr/codecs/vlen_utf8.py
index fb1fb76126..a10cb7c335 100644
--- a/src/zarr/codecs/vlen_utf8.py
+++ b/src/zarr/codecs/vlen_utf8.py
@@ -40,8 +40,7 @@ def to_dict(self) -> dict[str, JSON]:
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Self:
         return self
 
-    # TODO: expand the tests for this function
-    async def _decode_single(
+    def _decode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
@@ -55,7 +54,14 @@ async def _decode_single(
         as_string_dtype = decoded.astype(chunk_spec.dtype.to_native_dtype(), copy=False)
         return chunk_spec.prototype.nd_buffer.from_numpy_array(as_string_dtype)
 
-    async def _encode_single(
+    async def _decode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> NDBuffer:
+        return self._decode_sync(chunk_bytes, chunk_spec)
+
+    def _encode_sync(
         self,
         chunk_array: NDBuffer,
         chunk_spec: ArraySpec,
@@ -65,6 +71,13 @@ async def _encode_single(
             _vlen_utf8_codec.encode(chunk_array.as_numpy_array())
         )
 
+    async def _encode_single(
+        self,
+        chunk_array: NDBuffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return self._encode_sync(chunk_array, chunk_spec)
+
     def compute_encoded_size(self, input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         # what is input_byte_length for an object dtype?
         raise NotImplementedError("compute_encoded_size is not implemented for VLen codecs")
@@ -86,7 +99,7 @@ def to_dict(self) -> dict[str, JSON]:
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Self:
         return self
 
-    async def _decode_single(
+    def _decode_sync(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
@@ -99,7 +112,14 @@ async def _decode_single(
         decoded = _reshape_view(decoded, chunk_spec.shape)
         return chunk_spec.prototype.nd_buffer.from_numpy_array(decoded)
 
-    async def _encode_single(
+    async def _decode_single(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> NDBuffer:
+        return self._decode_sync(chunk_bytes, chunk_spec)
+
+    def _encode_sync(
         self,
         chunk_array: NDBuffer,
         chunk_spec: ArraySpec,
@@ -109,6 +129,13 @@ async def _encode_single(
             _vlen_bytes_codec.encode(chunk_array.as_numpy_array())
         )
 
+    async def _encode_single(
+        self,
+        chunk_array: NDBuffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return self._encode_sync(chunk_array, chunk_spec)
+
     def compute_encoded_size(self, input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         # what is input_byte_length for an object dtype?
         raise NotImplementedError("compute_encoded_size is not implemented for VLen codecs")
diff --git a/src/zarr/codecs/zstd.py b/src/zarr/codecs/zstd.py
index 27cc9a7777..f93c25a3c7 100644
--- a/src/zarr/codecs/zstd.py
+++ b/src/zarr/codecs/zstd.py
@@ -38,7 +38,7 @@ def parse_checksum(data: JSON) -> bool:
 class ZstdCodec(BytesBytesCodec):
     """zstd codec"""
 
-    is_fixed_size = True
+    is_fixed_size = False
 
     level: int = 0
     checksum: bool = False
@@ -71,23 +71,33 @@ def _zstd_codec(self) -> Zstd:
         config_dict = {"level": self.level, "checksum": self.checksum}
         return Zstd.from_config(config_dict)
 
+    def _decode_sync(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer:
+        return as_numpy_array_wrapper(self._zstd_codec.decode, chunk_bytes, chunk_spec.prototype)
+
     async def _decode_single(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer:
-        return await asyncio.to_thread(
-            as_numpy_array_wrapper, self._zstd_codec.decode, chunk_bytes, chunk_spec.prototype
-        )
+        return await asyncio.to_thread(self._decode_sync, chunk_bytes, chunk_spec)
+
+    def _encode_sync(
+        self,
+        chunk_bytes: Buffer,
+        chunk_spec: ArraySpec,
+    ) -> Buffer | None:
+        return as_numpy_array_wrapper(self._zstd_codec.encode, chunk_bytes, chunk_spec.prototype)
 
     async def _encode_single(
         self,
         chunk_bytes: Buffer,
         chunk_spec: ArraySpec,
     ) -> Buffer | None:
-        return await asyncio.to_thread(
-            as_numpy_array_wrapper, self._zstd_codec.encode, chunk_bytes, chunk_spec.prototype
-        )
+        return await asyncio.to_thread(self._encode_sync, chunk_bytes, chunk_spec)
 
     def compute_encoded_size(self, _input_byte_length: int, _chunk_spec: ArraySpec) -> int:
         raise NotImplementedError