[test] Make test object store more accurate.
diff --git a/release/benchmarks/object_store/test_object_store.py b/release/benchmarks/object_store/test_object_store.py
index 2403be078a079..ef8fb55340808 100644
--- a/release/benchmarks/object_store/test_object_store.py
+++ b/release/benchmarks/object_store/test_object_store.py
@@ -28,8 +28,8 @@ class Actor:
         def foo(self):
             pass
 
-        def sum(self, arr):
-            return np.sum(arr)
+        def data_len(self, arr):
+            return len(arr)
 
     actors = [Actor.remote() for _ in range(NUM_NODES)]
 
@@ -39,25 +39,28 @@ def sum(self, arr):
     for actor in tqdm(actors, desc="Ensure all actors have started."):
         ray.get(actor.foo.remote())
 
+    start = perf_counter()
     result_refs = []
     for actor in tqdm(actors, desc="Broadcasting objects"):
-        result_refs.append(actor.sum.remote(ref))
+        result_refs.append(actor.data_len.remote(ref))
 
     results = ray.get(result_refs)
+    end = perf_counter()
+
     for result in results:
         assert result == OBJECT_SIZE
 
+    return end - start
+
 
 ray.init(address="auto")
-start = perf_counter()
-test_object_broadcast()
-end = perf_counter()
-print(f"Broadcast time: {end - start} ({OBJECT_SIZE} B x {NUM_NODES} nodes)")
+duration = test_object_broadcast()
+print(f"Broadcast time: {duration} ({OBJECT_SIZE} B x {NUM_NODES} nodes)")
 
 if "TEST_OUTPUT_JSON" in os.environ:
     out_file = open(os.environ["TEST_OUTPUT_JSON"], "w")
     results = {
-        "broadcast_time": end - start,
+        "broadcast_time": duration,
         "object_size": OBJECT_SIZE,
         "num_nodes": NUM_NODES,
         "success": "1",
@@ -66,7 +69,7 @@ def sum(self, arr):
     results["perf_metrics"] = [
         {
             "perf_metric_name": perf_metric_name,
-            "perf_metric_value": end - start,
+            "perf_metric_value": duration,
             "perf_metric_type": "LATENCY",
         }
     ]

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## Why are these changes needed?
The test include ray.put and also actor start time. These should be excluded from the testing to make it more accurate.
<!-- Please give a short summary of the change and the problem this solves. -->

## Related issue number

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## Checks

- [ ] I've signed off every commit(by using the -s flag, i.e., `git commit -s`) in this PR.
- [ ] I've run `scripts/format.sh` to lint the changes in this PR.
- [ ] I've included any doc changes needed for https://docs.ray.io/en/master/.
    - [ ] I've added any new APIs to the API Reference. For example, if I added a 
           method in Tune, I've added it in `doc/source/tune/api/` under the 
           corresponding `.rst` file.
- [ ] I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [ ] Unit tests
   - [ ] Release tests
   - [ ] This PR is not tested :(
