Cache "is file" result as suggested.

src/watchdog/observers/read_directory_changes.py

@@ -44,11 +44,13 @@ def __init__(
         super().__init__(event_queue, watch, timeout=timeout, event_filter=event_filter)
         self._lock = threading.Lock()
         self._whandle: HANDLE | None = None
+        self._watched_files = {}
 
     def on_thread_start(self) -> None:
         watch_path = self.watch.path
         if os.path.isfile(watch_path):
-            watch_path = os.path.dirname(watch_path)
+            watch_path, basename = os.path.split(watch_path)
+            self._watched_files[self.watch.path] = basename
         self._whandle = get_directory_handle(watch_path)
 
     if platform.python_implementation() == "PyPy":
@@ -74,11 +76,12 @@ def queue_events(self, timeout: float) -> None:
         with self._lock:
             last_renamed_src_path = ""
             for winapi_event in winapi_events:
-                if os.path.isfile(self.watch.path):
-                    if os.path.basename(self.watch.path) != winapi_event.src_path:
+                try:
+                    basename = self._watched_files[self.watch.path]  # Is a file?
+                    if basename != winapi_event.src_path:
                         continue
                     src_path = self.watch.path
-                else:
+                except KeyError:
                     src_path = os.path.join(self.watch.path, winapi_event.src_path)
 
                 if winapi_event.is_renamed_old: