diff --git a/scripts/common.py b/scripts/common.py
index 50c90874d..1a88caaff 100644
--- a/scripts/common.py
+++ b/scripts/common.py
@@ -3036,12 +3036,16 @@ SYSTEM_ALIASES: dict[str, str] = {
 }
 
 
+@functools.lru_cache(maxsize=None)
 def _norm_system_id(sid: str) -> str:
     """Normalize system ID for cross-platform matching.
 
     Resolves known aliases, then strips manufacturer prefixes and separators
     so that platform-specific IDs (e.g., "xbox", "nintendo-wiiu") match
     profile IDs (e.g., "microsoft-xbox", "nintendo-wii-u").
+
+    A function of the string and two constant tables: a manifest run asks it
+    1.7 million times for about 1,100 ids.
     """
     s = sid.lower().replace("_", "-")
     s = SYSTEM_ALIASES.get(s, s)
diff --git a/tests/test_system_ids.py b/tests/test_system_ids.py
index cc054d902..1b8098c58 100644
--- a/tests/test_system_ids.py
+++ b/tests/test_system_ids.py
@@ -73,6 +73,18 @@ class PlatformNamesReachTheProfileSystem(unittest.TestCase):
             with self.subTest(alias=alias):
                 self.assertEqual(_norm_system_id(alias), _norm_system_id(canonical))
 
+    def test_each_id_is_normalized_once(self):
+        """A manifest run asked 1.7 million times for about a thousand ids,
+        1.5 seconds of prefix loops: the answer is kept per id."""
+        _norm_system_id.cache_clear()
+        for _ in range(3):
+            _norm_system_id("Nintendo_WiiU")
+        info = _norm_system_id.cache_info()
+        self.assertEqual((info.misses, info.hits), (1, 2))
+        self.assertEqual(
+            _norm_system_id("Nintendo_WiiU"), _norm_system_id.__wrapped__("Nintendo_WiiU")
+        )
+
 
 class AFoldedClaimKeepsItsProof(unittest.TestCase):
     """One spelling per machine folds bsnes-hd's SGB1.sfc onto bsnes's: the