diff --git a/homeassistant/helpers/event.py b/homeassistant/helpers/event.py
index 3d51610010c..f689f15725d 100644
--- a/homeassistant/helpers/event.py
+++ b/homeassistant/helpers/event.py
@@ -401,9 +401,6 @@ def _async_track_event(
     if not keys:
         return _remove_empty_listener
 
-    if isinstance(keys, str):
-        keys = [keys]
-
     hass_data = hass.data
     callbacks_key = tracker.callbacks_key
 
@@ -422,11 +419,23 @@ def _async_track_event(
 
     job = HassJob(action, f"track {tracker.event_type} event {keys}", job_type=job_type)
 
-    for key in keys:
-        if callback_list := callbacks.get(key):
+    if isinstance(keys, str):
+        # Almost all calls to this function use a single key
+        # so we optimize for that case. We don't use setdefault
+        # here because this function gets called ~20000 times
+        # during startup, and we want to avoid the overhead of
+        # creating empty lists and throwing them away.
+        if callback_list := callbacks.get(keys):
             callback_list.append(job)
         else:
-            callbacks[key] = [job]
+            callbacks[keys] = [job]
+        keys = [keys]
+    else:
+        for key in keys:
+            if callback_list := callbacks.get(key):
+                callback_list.append(job)
+            else:
+                callbacks[key] = [job]
 
     return ft.partial(_remove_listener, hass, listeners_key, keys, job, callbacks)