diff --git a/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetBuilder.java b/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetBuilder.java
index 3cb388f..cb27452 100644
--- a/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetBuilder.java
@@ -199,7 +199,7 @@
   }
 
   private static final ConcurrentMap<ImmutableList<?>, NestedSet<?>> immutableListCache =
-      new MapMaker().weakKeys().makeMap();
+      new MapMaker().concurrencyLevel(16).weakKeys().makeMap();
 
   /**
    * Creates a nested set from a given list of items.