diff --git a/src/workerd/io/observer.h b/src/workerd/io/observer.h
index 9b439fd6a49..e71a561f3b4 100644
--- a/src/workerd/io/observer.h
+++ b/src/workerd/io/observer.h
@@ -109,6 +109,10 @@ class IsolateObserver: public kj::AtomicRefcounted {
 
   class LockTiming {
   public:
+    virtual void waitingForOtherIsolate(kj::StringPtr id) {}
+    // Called by `Isolate::takeAsyncLock()` when it is blocked by a different isolate lock on the
+    // same thread.
+
     virtual void reportAsyncInfo(uint currentLoad, bool threadWaitingSameLock,
         uint threadWaitingDifferentLockCount) {}
     // Call if this is an async lock attempt, before constructing LockRecord.
diff --git a/src/workerd/io/worker.c++ b/src/workerd/io/worker.c++
index 054469fc550..fd7952ebd55 100644
--- a/src/workerd/io/worker.c++
+++ b/src/workerd/io/worker.c++
@@ -1780,6 +1780,9 @@ kj::Promise<Worker::AsyncLock> Worker::Isolate::takeAsyncLockImpl(
       // be released before we try to lock a different isolate.
       // TODO(perf): Use of ForkedPromise leads to thundering herd here. Should be minor in practice,
       //   but we could consider creating another linked list instead...
+      KJ_IF_MAYBE(lt, lockTiming) {
+        lt->get()->waitingForOtherIsolate(waiter->isolate->getId());
+      }
       co_await waiter->releasePromise.addBranch();
     }
   }