diff --git a/src/ophyd_async/core/_device.py b/src/ophyd_async/core/_device.py
index 365e5714a..7b331ac2f 100644
--- a/src/ophyd_async/core/_device.py
+++ b/src/ophyd_async/core/_device.py
@@ -5,7 +5,7 @@
 from collections.abc import Coroutine, Iterator, Mapping, MutableMapping
 from functools import cached_property
 from logging import LoggerAdapter, getLogger
-from typing import Any, Literal, TypeVar
+from typing import Any, TypeVar
 
 from bluesky.protocols import HasName
 from bluesky.run_engine import call_in_bluesky_event_loop, in_bluesky_event_loop
@@ -63,11 +63,8 @@ class Device(HasName, Connectable):
     parent: Device | None = None
     # None if connect hasn't started, a Task if it has
     _connect_task: asyncio.Task | None = None
-    _mock: (
-        None  # If we have never connected
-        | LazyMock  # The mock if we have connected in mock mode
-        | Literal[False]  # If we have not connected in mock mode
-    ) = None
+    # The mock if we have connected in mock mode
+    _mock: LazyMock | None = None
 
     def __init__(
         self, name: str = "", connector: DeviceConnector | None = None
@@ -111,16 +108,20 @@ def set_name(self, name: str):
             child.set_name(child_name)
 
     def __setattr__(self, name: str, value: Any) -> None:
+        # Bear in mind that this function is called *a lot*, so
+        # we need to make sure nothing expensive happens in it...
         if name == "parent":
             if self.parent not in (value, None):
                 raise TypeError(
                     f"Cannot set the parent of {self} to be {value}: "
                     f"it is already a child of {self.parent}"
                 )
+        # ...hence not doing an isinstance check for attributes we
+        # know not to be Devices
         elif name not in _not_device_attrs and isinstance(value, Device):
             value.parent = self
             self._child_devices[name] = value
-        # Avoid the super call as this happens a lot
+        # ...and avoiding the super call as we know it resolves to `object`
         return object.__setattr__(self, name, value)
 
     async def connect(
@@ -157,7 +158,7 @@ async def connect(
                 and not (self._connect_task.done() and self._connect_task.exception())
             )
             if force_reconnect or not can_use_previous_connect:
-                self._mock = False
+                self._mock = None
                 coro = self._connector.connect_real(self, timeout, force_reconnect)
                 self._connect_task = asyncio.create_task(coro)
             assert self._connect_task, "Connect task not created, this shouldn't happen"
diff --git a/src/ophyd_async/core/_utils.py b/src/ophyd_async/core/_utils.py
index d546cb7d5..ca20d90a3 100644
--- a/src/ophyd_async/core/_utils.py
+++ b/src/ophyd_async/core/_utils.py
@@ -265,6 +265,24 @@ def __call__(self) -> T:
 
 
 class LazyMock:
+    """A lazily created Mock to be used when connecting in mock mode.
+
+    Creating Mocks is reasonably expensive when each Device (and Signal)
+    requires its own, and the tree is only used when ``Signal.set()`` is
+    called. This class allows a tree of lazily connected Mocks to be
+    constructed so that when the leaf is created, so are its parents.
+    Any calls to the child are then accessible from the parent mock.
+
+    >>> parent = LazyMock()
+    >>> child = parent.child("child")
+    >>> child_mock = child()
+    >>> child_mock()  # doctest: +ELLIPSIS
+    <Mock name='mock.child()' id='...'>
+    >>> parent_mock = parent()
+    >>> parent_mock.mock_calls
+    [call.child()]
+    """
+
     def __init__(self, name: str = "", parent: LazyMock | None = None) -> None:
         self.parent = parent
         self.name = name
diff --git a/tests/core/test_device.py b/tests/core/test_device.py
index 70620a0bf..1f9c0ffb8 100644
--- a/tests/core/test_device.py
+++ b/tests/core/test_device.py
@@ -175,11 +175,16 @@ def __init__(self, name: str) -> None:
         super().__init__(name)
 
 
-async def test_many_individual_device_connects_not_slow():
+@pytest.mark.parametrize("parallel", (False, True))
+async def test_many_individual_device_connects_not_slow(parallel):
     start = time.time()
-    for i in range(100):
-        bundle = MotorBundle(f"bundle{i}")
-        await bundle.connect(mock=True)
+    bundles = [MotorBundle(f"bundle{i}") for i in range(200)]
+    if parallel:
+        for bundle in bundles:
+            await bundle.connect(mock=True)
+    else:
+        coros = {bundle.name: bundle.connect(mock=True) for bundle in bundles}
+        await wait_for_connection(**coros)
     duration = time.time() - start
     assert duration < 1