# UNDF: UNDF-2026-000000060
--- a/src/mod_mam.erl
+++ b/src/mod_mam.erl
@@ -1021,14 +1021,14 @@ check_store_hint(Pkt) ->
 -spec should_archive_peer(binary(), binary(),
 			  #archive_prefs{}, jid()) -> boolean().
 should_archive_peer(LUser, LServer,
-		    #archive_prefs{default = Default,
-				   always = Always,
-				   never = Never},
+		    #archive_prefs{default = Default,
+				   always = AlwaysList,
+				   never  = NeverList},
 		    Peer) ->
     LPeer = jid:remove_resource(jid:tolower(Peer)),
-    case lists:member(LPeer, Always) of
+    Always = gb_sets:from_list(AlwaysList),
+    Never  = gb_sets:from_list(NeverList),
+    case gb_sets:is_member(LPeer, Always) of
 	true ->
 	    true;
 	false ->
-	    case lists:member(LPeer, Never) of
+	    case gb_sets:is_member(LPeer, Never) of
 		true ->
 		    false;
 		false ->
@@ -1194,8 +1194,10 @@ check_store_hint(Pkt) ->
 write_prefs(LUser, LServer, Host, Default, Always, Never) ->
+    %% Always/Never arrive as sorted lists (lists:usort applied by caller).
+    %% Store as lists for serialisation; convert to gb_sets in hot path.
     Prefs = #archive_prefs{us = {LUser, LServer},
 			   default = Default,
-			   always = Always,
-			   never = Never},
+			   always = gb_sets:to_list(gb_sets:from_list(Always)),
+			   never  = gb_sets:to_list(gb_sets:from_list(Never))},