Add comment to explain not using the core API in MQTT client (#108942)

homeassistant/components/mqtt/client.py

@@ -216,6 +216,10 @@ def subscribe(
 
     def remove() -> None:
         """Remove listener convert."""
+        # MQTT messages tend to be high volume,
+        # and since they come in via a thread and need to be processed in the event loop,
+        # we want to avoid hass.add_job since most of the time is spent calling
+        # inspect to figure out how to run the callback.
         hass.loop.call_soon_threadsafe(async_remove)
 
     return remove
@@ -796,6 +800,10 @@ class MQTT:
         self, _mqttc: mqtt.Client, _userdata: None, msg: mqtt.MQTTMessage
     ) -> None:
         """Message received callback."""
+        # MQTT messages tend to be high volume,
+        # and since they come in via a thread and need to be processed in the event loop,
+        # we want to avoid hass.add_job since most of the time is spent calling
+        # inspect to figure out how to run the callback.
         self.loop.call_soon_threadsafe(self._mqtt_handle_message, msg)
 
     @lru_cache(None)  # pylint: disable=method-cache-max-size-none