diff --git a/electrum/transaction.py b/electrum/transaction.py
index cca06ff189db..212c861580cf 100644
--- a/electrum/transaction.py
+++ b/electrum/transaction.py
@@ -625,7 +625,8 @@ def read_uint64(self): return self._read_num('<Q')
 
     def write_boolean(self, val): return self.write(b'\x01' if val else b'\x00')
     def write_int16(self, val): return self._write_num('<h', val)
-    def write_uint16(self, val): return self._write_num('<H', val)
+    def write_uint16(self, val):
+        return self._write_num('<H', val)
     def write_int32(self, val): return self._write_num('<i', val)
     def write_uint32(self, val): return self._write_num('<I', val)
     def write_int64(self, val): return self._write_num('<q', val)
@@ -672,7 +673,15 @@ def _read_num(self, format):
 
     def _write_num(self, format, num):
         s = struct.pack(format, num)
-        self.write(s)
+        inp = self.input
+        if inp is None:
+            # Direct assignment is fastest
+            self.input = bytearray(s)
+        else:
+            # Use extend() instead of += for efficiency (avoids creating a new object)
+            inp.extend(s)
+        # Return behavior preserved: The original _write_num doesn't return, but mainline code expects None
+        # which is the case here.
 
 
 def script_GetOp(_bytes : bytes):