diff --git a/demo/benchmark_large_house.mjs b/demo/benchmark_large_house.mjs
index f7d8634d..c6b7a2be 100644
--- a/demo/benchmark_large_house.mjs
+++ b/demo/benchmark_large_house.mjs
@@ -294,6 +294,26 @@ try {
       } else history.replaceState(null, '', location.pathname);
       const host = document.getElementById('host');
       const card = document.createElement('houseplan-card');
+      // #520 diagnostics: where the extra half second of model readiness goes.
+      // Counts Lit update cycles and model builds; printed, never budgeted.
+      card.__diag = { updates: 0, updateMs: 0, models: 0, adopts: 0 };
+      const diagPerform = card.performUpdate.bind(card);
+      card.performUpdate = function () {
+        const started = performance.now();
+        const result = diagPerform();
+        card.__diag.updates += 1;
+        card.__diag.updateMs += performance.now() - started;
+        return result;
+      };
+      const diagBuild = card._buildModel.bind(card);
+      card._buildModel = function () { card.__diag.models += 1; return diagBuild(); };
+      if (typeof card._adoptStructuralResponses === 'function') {
+        const legacy = card._adoptStructuralResponses.bind(card);
+        card._adoptStructuralResponses = function (...args) { card.__diag.adopts += 1; return legacy(...args); };
+      } else if (typeof card._adoptAuthoritative === 'function') {
+        const gated = card._adoptAuthoritative.bind(card);
+        card._adoptAuthoritative = function (...args) { card.__diag.adopts += 1; return gated(...args); };
+      }
       card.setConfig({
         type: 'custom:houseplan-card', title: `Performance baseline ${sample}`, icon_size: 3.4,
       });
@@ -363,6 +383,10 @@ try {
       if (interaction && '_bootSoft' in card) await until(() => card._bootSoft === false);
       await frame();
       const firstStableRenderMs = Number((performance.now() - loadStarted).toFixed(2));
+      console.log(`#520 diag sample ${sample}: updates=${card.__diag.updates}`
+        + ` updateMs=${card.__diag.updateMs.toFixed(1)} models=${card.__diag.models}`
+        + ` adopts=${card.__diag.adopts} cfgEpoch=${card._cfgEpoch}`
+        + ` modelReady=${modelReadyMs} firstStable=${firstStableRenderMs}`);
       const initialProjection = typeof card._effectiveProjection === 'function'
         ? card._effectiveProjection() : null;
       if (requiresIsometric && initialProjection !== 'iso')
diff --git a/node_modules b/node_modules
new file mode 120000
index 00000000..824938e6
--- /dev/null
+++ b/node_modules
@@ -0,0 +1 @@
+/tmp/rel/node_modules
\ No newline at end of file