diff --git a/src/main/org/apache/tools/ant/types/resources/Union.java b/src/main/org/apache/tools/ant/types/resources/Union.java
index 9c1edb8b2..c0c72b259 100644
--- a/src/main/org/apache/tools/ant/types/resources/Union.java
+++ b/src/main/org/apache/tools/ant/types/resources/Union.java
@@ -104,14 +104,15 @@ public class Union extends BaseResourceCollectionContainer {
         //preserve order-encountered using a list; enforce set logic manually:
         // (LinkedHashSet better, but JDK 1.4+)
         ArrayList union = new ArrayList(rc.size() * 2);
-        Set _union = new HashSet(rc.size() * 2);
+        // Use a set as list.contains() can be expensive for lots of resources
+        Set set = new HashSet(rc.size() * 2);
         for (Iterator rcIter = rc.iterator(); rcIter.hasNext();) {
             for (Iterator r = nextRC(rcIter).iterator(); r.hasNext();) {
                 Object o = r.next();
                 if (asString) {
                     o = o.toString();
                 }
-                if (_union.add(o)) {
+                if (set.add(o)) {
                     union.add(o);
                 }
             }