diff --git a/lighthouse-cli/test/smokehouse/test-definitions/dobetterweb/dbw-expectations.js b/lighthouse-cli/test/smokehouse/test-definitions/dobetterweb/dbw-expectations.js
index b1f889685124..bc80333978f8 100644
--- a/lighthouse-cli/test/smokehouse/test-definitions/dobetterweb/dbw-expectations.js
+++ b/lighthouse-cli/test/smokehouse/test-definitions/dobetterweb/dbw-expectations.js
@@ -20,6 +20,7 @@ const expectations = {
     length: 28,
   },
   artifacts: {
+    BenchmarkIndex: '<10000',
     HostFormFactor: 'desktop',
     Stacks: [{
       id: 'jquery',
diff --git a/lighthouse-core/lib/page-functions.js b/lighthouse-core/lib/page-functions.js
index 3653d7c33b74..94b5937f2966 100644
--- a/lighthouse-core/lib/page-functions.js
+++ b/lighthouse-core/lib/page-functions.js
@@ -208,7 +208,8 @@ function computeBenchmarkIndex() {
 
     while (Date.now() - start < 500) {
       let s = '';
-      for (let j = 0; j < 10000; j++) s += 'a'; // eslint-disable-line no-unused-vars
+      for (let j = 0; j < 10000; j++) s += 'a';
+      if (s.length === 1) throw new Error('will never happen, but prevents compiler optimizations');
 
       iterations++;
     }