diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index 428e6dec6749..d7388e3bbdf5 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -89,6 +89,17 @@
 
 
 class SubtypeContext:
+    __slots__ = (
+        "ignore_type_params",
+        "ignore_pos_arg_names",
+        "ignore_declared_variance",
+        "always_covariant",
+        "ignore_promotions",
+        "erase_instances",
+        "keep_erased_types",
+        "options",
+    )
+
     def __init__(
         self,
         *,