diff --git a/core/src/apps/bitcoin/sign_tx/__init__.py b/core/src/apps/bitcoin/sign_tx/__init__.py
index f0c0b99bb..396195477 100644
--- a/core/src/apps/bitcoin/sign_tx/__init__.py
+++ b/core/src/apps/bitcoin/sign_tx/__init__.py
@@ -39,12 +39,13 @@ async def sign_tx(
     signer = signer_class(msg, keychain, coin).signer()
 
     res = None  # type: Union[TxAck, bool, None]
+    field_cache = {}
     while True:
         req = signer.send(res)
         if isinstance(req, TxRequest):
             if req.request_type == TXFINISHED:
                 break
-            res = await ctx.call(req, TxAck)
+            res = await ctx.call(req, TxAck, field_cache)
         elif isinstance(req, helpers.UiConfirmOutput):
             mods = utils.unimport_begin()
             res = await layout.confirm_output(ctx, req.output, req.coin)
diff --git a/core/src/protobuf.py b/core/src/protobuf.py
index ff568d44d..78ab23997 100644
--- a/core/src/protobuf.py
+++ b/core/src/protobuf.py
@@ -185,9 +185,16 @@ if False:
 
 
 def load_message(
-    reader: Reader, msg_type: Type[LoadedMessageType]
+    reader: Reader, msg_type: Type[LoadedMessageType], field_cache: Dict = None
 ) -> LoadedMessageType:
-    fields = msg_type.get_fields()
+
+    if field_cache is None:
+        field_cache = {}
+    fields = field_cache.get(msg_type)
+    if fields is None:
+        fields = msg_type.get_fields()
+        field_cache[msg_type] = fields
+
     msg = msg_type()
 
     if False:
@@ -238,7 +245,7 @@ def load_message(
             reader.readinto(fvalue)
             fvalue = bytes(fvalue).decode()
         elif issubclass(ftype, MessageType):
-            fvalue = load_message(LimitedReader(reader, ivalue), ftype)
+            fvalue = load_message(LimitedReader(reader, ivalue), ftype, field_cache)
         else:
             raise TypeError  # field type is unknown
 
@@ -257,11 +264,15 @@ def load_message(
     return msg
 
 
-def dump_message(writer: Writer, msg: MessageType, fields: Dict = None) -> None:
+def dump_message(writer: Writer, msg: MessageType, field_cache: Dict = None) -> None:
     repvalue = [0]
 
+    if field_cache is None:
+        field_cache = {}
+    fields = field_cache.get(type(msg))
     if fields is None:
         fields = msg.get_fields()
+        field_cache[type(msg)] = fields
 
     for ftag in fields:
         fname, ftype, fflags = fields[ftag]
@@ -276,8 +287,6 @@ def dump_message(writer: Writer, msg: MessageType, fields: Dict = None) -> None:
             repvalue[0] = fvalue
             fvalue = repvalue
 
-        ffields = None  # type: Optional[Dict]
-
         for svalue in fvalue:
             dump_uvarint(writer, fkey)
 
@@ -308,21 +317,27 @@ def dump_message(writer: Writer, msg: MessageType, fields: Dict = None) -> None:
                 writer.write(svalue)
 
             elif issubclass(ftype, MessageType):
+                ffields = field_cache.get(ftype)
                 if ffields is None:
                     ffields = ftype.get_fields()
-                dump_uvarint(writer, count_message(svalue, ffields))
-                dump_message(writer, svalue, ffields)
+                    field_cache[ftype] = ffields
+                dump_uvarint(writer, count_message(svalue, field_cache))
+                dump_message(writer, svalue, field_cache)
 
             else:
                 raise TypeError
 
 
-def count_message(msg: MessageType, fields: Dict = None) -> int:
+def count_message(msg: MessageType, field_cache: Dict = None) -> int:
     nbytes = 0
     repvalue = [0]
 
+    if field_cache is None:
+        field_cache = {}
+    fields = field_cache.get(type(msg))
     if fields is None:
         fields = msg.get_fields()
+        field_cache[msg] = fields
 
     for ftag in fields:
         fname, ftype, fflags = fields[ftag]
diff --git a/core/src/trezor/wire/__init__.py b/core/src/trezor/wire/__init__.py
index f79b5cc16..edcfd7ddc 100644
--- a/core/src/trezor/wire/__init__.py
+++ b/core/src/trezor/wire/__init__.py
@@ -141,11 +141,14 @@ class Context:
         self.buffer_io = codec_v1.BytesIO(bytearray(8192))
 
     async def call(
-        self, msg: protobuf.MessageType, expected_type: Type[protobuf.LoadedMessageType]
+        self,
+        msg: protobuf.MessageType,
+        expected_type: Type[protobuf.LoadedMessageType],
+        field_cache: Dict = None,
     ) -> protobuf.LoadedMessageType:
-        await self.write(msg)
+        await self.write(msg, field_cache)
         del msg
-        return await self.read(expected_type)
+        return await self.read(expected_type, field_cache)
 
     async def call_any(
         self, msg: protobuf.MessageType, *expected_wire_types: int
@@ -159,7 +162,7 @@ class Context:
         return await codec_v1.read_message(self.iface, self.buffer_io.buffer)
 
     async def read(
-        self, expected_type: Type[protobuf.LoadedMessageType]
+        self, expected_type: Type[protobuf.LoadedMessageType], field_cache: Dict = None
     ) -> protobuf.LoadedMessageType:
         if __debug__:
             log.debug(
@@ -191,7 +194,7 @@ class Context:
 
         # look up the protobuf class and parse the message
         pbtype = messages.get_type(msg.type)
-        return protobuf.load_message(msg.data, pbtype)
+        return protobuf.load_message(msg.data, pbtype, field_cache)
 
     async def read_any(
         self, expected_wire_types: Iterable[int]
@@ -226,7 +229,7 @@ class Context:
         # parse the message and return it
         return protobuf.load_message(msg.data, exptype)
 
-    async def write(self, msg: protobuf.MessageType) -> None:
+    async def write(self, msg: protobuf.MessageType, field_cache: Dict = None) -> None:
         if __debug__:
             log.debug(
                 __name__, "%s:%x write: %s", self.iface.iface_num(), self.sid, msg
@@ -234,7 +237,7 @@ class Context:
 
         # write the message
         self.buffer_io.seek(0)
-        protobuf.dump_message(self.buffer_io, msg)
+        protobuf.dump_message(self.buffer_io, msg, field_cache)
         await codec_v1.write_message(
             self.iface, msg.MESSAGE_WIRE_TYPE, self.buffer_io.get_written()
         )