diff --git a/scripts/common.py b/scripts/common.py
index 35b53465..249b65a2 100644
--- a/scripts/common.py
+++ b/scripts/common.py
@@ -1018,13 +1018,32 @@ def _rank_tail_candidates(
     )
 
 
+# id(files) -> (files, len(files), {archive: entries with a crc32}). Callers
+# ask once per entry of the same profile: scanning the list each time walked
+# 38 million entries in one verify --all over mame.yml. The list is held so
+# its id is not reused, a list that changed length is indexed again, and the
+# members are read from the entries at each call. Short lists are scanned
+# directly: a fresh default [] per call would only fill the cache.
+_ARCHIVE_INDEX_MIN = 64
+_archive_index: dict[int, tuple[list[dict], int, dict[str | None, list[dict]]]] = {}
+
+
+def _member(f: dict) -> dict:
+    return {"crc32": str(f["crc32"]).lower(), "required": bool(f.get("required", True))}
+
+
 def archive_members(archive: str, files: list[dict]) -> list[dict]:
     """The members a profile reads inside *archive*, with what it requires."""
-    return [
-        {"crc32": str(f["crc32"]).lower(), "required": bool(f.get("required", True))}
-        for f in files
-        if f.get("archive") == archive and f.get("crc32")
-    ]
+    if len(files) < _ARCHIVE_INDEX_MIN:
+        return [_member(f) for f in files if f.get("archive") == archive and f.get("crc32")]
+    cached = _archive_index.get(id(files))
+    if cached is None or cached[0] is not files or cached[1] != len(files):
+        index: dict[str | None, list[dict]] = {}
+        for f in files:
+            if f.get("crc32"):
+                index.setdefault(f.get("archive"), []).append(f)
+        cached = _archive_index[id(files)] = (files, len(files), index)
+    return [_member(f) for f in cached[2].get(archive, ())]
 
 
 def archive_entry(archive: str, files: list[dict]) -> dict:
diff --git a/tests/test_archive_choice.py b/tests/test_archive_choice.py
index 046742d3..039d11d2 100644
--- a/tests/test_archive_choice.py
+++ b/tests/test_archive_choice.py
@@ -409,5 +409,50 @@ class ALooseEntryNamingItsArchive(unittest.TestCase):
         self.assertEqual(report["total_files"], 1)
 
 
+class ArchiveMembersIndexOnce(unittest.TestCase):
+    """with_archive_members asks once per entry of a profile: scanning the
+    whole list for each walked 38 million entries in one verify --all."""
+
+    class CountingList(list):
+        walks = 0
+
+        def __iter__(self):
+            type(self).walks += 1
+            return super().__iter__()
+
+    def entries(self) -> list[dict]:
+        files = self.CountingList()
+        self.CountingList.walks = 0
+        for n in range(300):
+            files.append({"name": f"m{n}.rom", "archive": f"set{n % 30}.zip",
+                          "crc32": f"{n:08X}", "required": n % 3 != 0})
+        files.append({"name": "loose.bin", "crc32": "deadbeef"})
+        files.append({"name": "nocrc.rom", "archive": "set0.zip"})
+        return files
+
+    @staticmethod
+    def scanned(archive, files) -> list[dict]:
+        return [{"crc32": str(f["crc32"]).lower(), "required": bool(f.get("required", True))}
+                for f in list.__iter__(files) if f.get("archive") == archive and f.get("crc32")]
+
+    def test_a_profile_list_is_walked_once_and_answers_as_a_scan(self):
+        from common import archive_members
+
+        files = self.entries()
+        for archive in [f"set{n}.zip" for n in range(30)] + ["absent.zip", "", None]:
+            with self.subTest(archive=archive):
+                self.assertEqual(archive_members(archive, files), self.scanned(archive, files))
+        self.assertEqual(self.CountingList.walks, 1)
+
+    def test_a_list_that_grew_is_indexed_again(self):
+        from common import archive_members
+
+        files = self.entries()
+        archive_members("set1.zip", files)
+        files.append({"name": "late.rom", "archive": "set1.zip", "crc32": "ABCDEF01"})
+        self.assertEqual(archive_members("set1.zip", files), self.scanned("set1.zip", files))
+        self.assertIn({"crc32": "abcdef01", "required": True}, archive_members("set1.zip", files))
+
+
 if __name__ == "__main__":
     unittest.main()