diff --git a/changes/4081-samuelcolvin.md b/changes/4081-samuelcolvin.md
new file mode 100644
index 0000000..53fd4f5
--- /dev/null
+++ b/changes/4081-samuelcolvin.md
@@ -0,0 +1 @@
+Speedup `__isinstancecheck__` on pydantic models when the type is not a model, may also avoid memory "leaks".
diff --git a/pydantic/main.py b/pydantic/main.py
index 7afcafc..0c20d9e 100644
--- a/pydantic/main.py
+++ b/pydantic/main.py
@@ -295,6 +295,14 @@ class ModelMetaclass(ABCMeta):
 
         return cls
 
+    def __instancecheck__(self, instance: Any) -> bool:
+        """
+        Avoid calling ABC _abc_subclasscheck unless we're pretty sure.
+
+        See #3829 and python/cpython#92810
+        """
+        return hasattr(instance, '__fields__') and super().__instancecheck__(instance)
+
 
 object_setattr = object.__setattr__
 
diff --git a/tests/test_edge_cases.py b/tests/test_edge_cases.py
index 5da6225..b7083d0 100644
--- a/tests/test_edge_cases.py
+++ b/tests/test_edge_cases.py
@@ -1932,3 +1932,17 @@ def test_int_subclass():
 
     m = MyModel(my_int=IntSubclass(123))
     assert m.my_int.__class__ == IntSubclass
+
+
+def test_model_issubclass():
+    assert not issubclass(int, BaseModel)
+
+    class MyModel(BaseModel):
+        x: int
+
+    assert issubclass(MyModel, BaseModel)
+
+    class Custom:
+        __fields__ = True
+
+    assert not issubclass(Custom, BaseModel)