diff --git a/lib/page.js b/lib/page.js
index 75312ca33d52..0041c699abbc 100644
--- a/lib/page.js
+++ b/lib/page.js
@@ -18,6 +18,12 @@ import getLinkData from './get-link-data.js'
 import getDocumentType from './get-document-type.js'
 import { union } from 'lodash-es'
 
+// We're going to check a lot of pages' "ID" (the first part of
+// the relativePath) against `productMap` to make sure it's valid.
+// To avoid having to do `Object.keys(productMap).includes(id)`
+// every single time, we turn it into a Set once.
+const productMapKeysAsSet = new Set(Object.keys(productMap))
+
 // Wrapper on renderContent() that caches the output depending on the
 // `context` by extracting information about the page's current permalink
 const _renderContentCache = new Map()
@@ -162,10 +168,7 @@ class Page {
 
     // make sure the ID is valid
     if (process.env.NODE_ENV !== 'test') {
-      assert(
-        Object.keys(productMap).includes(id),
-        `page ${this.fullPath} has an invalid product ID: ${id}`
-      )
+      assert(productMapKeysAsSet.has(id), `page ${this.fullPath} has an invalid product ID: ${id}`)
     }
 
     return id
diff --git a/tests/content/site-data.js b/tests/content/site-data.js
index 40398c7dd4aa..469885e456ca 100644
--- a/tests/content/site-data.js
+++ b/tests/content/site-data.js
@@ -48,15 +48,19 @@ describe('siteData module (English)', () => {
     expect(reusable.includes('任意のページの左上で')).toBe(true)
   })
 
-  // TODO: re-enable once Janky flakyness is resolved
-  // Docs Engineering issue: 964
-  test.skip('backfills missing translated site data with English values', async () => {
+  test('backfills missing translated site data with English values', async () => {
     const newFile = path.join(__dirname, '../../data/newfile.yml')
-    await fs.writeFile(newFile, 'newvalue: bar')
-    const data = await loadSiteData()
-    expect(get(data, 'en.site.data.newfile.newvalue')).toEqual('bar')
-    expect(get(data, 'ja.site.data.newfile.newvalue')).toEqual('bar')
-    await fs.unlink(newFile)
+    fs.writeFileSync(newFile, 'newvalue: bar')
+    try {
+      const data = loadSiteData()
+      expect(get(data, 'en.site.data.newfile.newvalue')).toEqual('bar')
+      expect(get(data, 'ja.site.data.newfile.newvalue')).toEqual('bar')
+    } finally {
+      // If an error is thrown above, it will still "bubble up"
+      // to the jest reporter, but we still always need to clean up
+      // the temporary file.
+      fs.unlinkSync(newFile)
+    }
   })
 
   test('all Liquid templating is valid', async () => {