# UNDF: UNDF-2026-000000743
# UNDF: (leave blank)
--- a/xds/src/main/java/io/grpc/xds/client/XdsClientImpl.java
+++ b/xds/src/main/java/io/grpc/xds/client/XdsClientImpl.java
@@ -1091,9 +1091,7 @@ public final class XdsClientImpl extends XdsClient {
   private Collection<String> getActiveAuthorities(ControlPlaneClient cpc) {
-    List<String> asList = activatedCpClients.entrySet().stream()
+    // Always return a HashSet for O(1) contains(); linear scan over asList was O(A×S×T)
+    // inside double-loops in cleanUpResourceTimers and onControlPlaneClientError.
+    return activatedCpClients.entrySet().stream()
         .filter(entry -> !entry.getValue().isEmpty()
             && cpc == entry.getValue().get(entry.getValue().size() - 1))
         .map(Map.Entry::getKey)
-        .collect(Collectors.toList());
-
-    // Since this is usually used for contains, use a set when the list is large
-    return (asList.size() < 100) ? asList : new HashSet<>(asList);
+        .collect(Collectors.toCollection(HashSet::new));
   }