diff --git a/test/core/promise/activity_test.cc b/test/core/promise/activity_test.cc
index 0fcd9422f83..dfd62ed2f61 100644
--- a/test/core/promise/activity_test.cc
+++ b/test/core/promise/activity_test.cc
@@ -359,7 +359,7 @@ TEST(AtomicWakerTest, ThreadStress) {
   std::atomic<int> not_armed{0};
   AtomicWaker waker;
 
-  threads.reserve(90);
+  threads.reserve(15);
   for (int i = 0; i < 5; i++) {
     threads.emplace_back([&] {
       while (!done.load(std::memory_order_relaxed)) {