diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7f0b268c9..42c12294b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,9 @@ and this project uses [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 
 ### Fixed
 
+- Psychometric observation replacement now removes only the existing row's
+  contiguous trailing items instead of scanning the complete response ledger;
+  the checked-in benchmark reports fit/rank and observation p50/p95 separately.
 - Workflow workers now preserve the caller message array exactly once, while
   the added envelope carries only the subtask and Conductor-style prior-step
   access list instead of duplicating the task or source attachments.
diff --git a/contextual_orchestrator/psychometric_routing.py b/contextual_orchestrator/psychometric_routing.py
index bef8645fc..7ab731647 100644
--- a/contextual_orchestrator/psychometric_routing.py
+++ b/contextual_orchestrator/psychometric_routing.py
@@ -61,13 +61,10 @@ def observe_context_id(
             values = (int(accepted), *(int(value) for value in irt_row))
             if any(value not in (0, 1) for value in values):
                 raise ValueError("judge IRT rows must be dichotomous")
-            stale = [
-                key
-                for key in self._responses
-                if key[:2] == (agent_id, context_id) and key[2] >= len(values)
-            ]
-            for key in stale:
-                del self._responses[key]
+            stale_index = len(values)
+            while (agent_id, context_id, stale_index) in self._responses:
+                del self._responses[(agent_id, context_id, stale_index)]
+                stale_index += 1
             for item_index, value in enumerate(values):
                 self._responses[(agent_id, context_id, item_index)] = value
             while len(self._contexts) > self.max_contexts:
diff --git a/docs/doctoring/measured-routing-evidence.md b/docs/doctoring/measured-routing-evidence.md
index db6a91ed1..b44669d29 100644
--- a/docs/doctoring/measured-routing-evidence.md
+++ b/docs/doctoring/measured-routing-evidence.md
@@ -57,6 +57,12 @@ latency or answer accuracy. The next accuracy experiment must use a held-out
 model-query matrix and report log loss or Brier score alongside routing regret;
 true-parameter simulations must continue to report RMSE.
 
+The successor observation-path experiment uses the same 512-context ledger.
+Replacing one model/context row fell from p50 0.133833 ms and p95 0.152166 ms
+on `b2f90116` to p50 0.000875 ms and p95 0.001000 ms. The benchmark now emits
+both fields. This is local gateway bookkeeping evidence; the fit, held-out
+quality, and provider latency remain separate KPIs.
+
 ## APA 7 references
 
 Chen, L., Zaharia, M., & Zou, J. (2023). *FrugalGPT: How to use large
diff --git a/scripts/benchmark_psychometric_routing.py b/scripts/benchmark_psychometric_routing.py
index 4b16fb7ef..1fcc0af36 100644
--- a/scripts/benchmark_psychometric_routing.py
+++ b/scripts/benchmark_psychometric_routing.py
@@ -65,6 +65,19 @@ def main() -> None:
             samples_ms.append((time.perf_counter_ns() - started_ns) / 1_000_000)
 
     assert len(ranked) == len(model_ids)
+    observation_samples_ms: list[float] = []
+    for sample_index in range(101):
+        started_ns = time.perf_counter_ns()
+        evidence.observe(
+            "context_511",
+            "model_3",
+            bool(sample_index % 2),
+            None,
+            irt_row=(int(not sample_index % 2),),
+        )
+        observation_samples_ms.append(
+            (time.perf_counter_ns() - started_ns) / 1_000_000
+        )
     print(
         json.dumps(
             {
@@ -72,6 +85,8 @@ def main() -> None:
                 "models": len(model_ids),
                 "items_per_context": 2,
                 "median_fit_and_rank_ms": statistics.median(samples_ms),
+                "median_observe_ms": statistics.median(observation_samples_ms),
+                "p95_observe_ms": sorted(observation_samples_ms)[95],
                 "samples_ms": samples_ms,
             },
             sort_keys=True,