diff --git a/core/modules/media/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php b/core/modules/media/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
index 071b4121c8..5bd2990e40 100644
--- a/core/modules/media/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
@@ -243,14 +243,13 @@ public function testPreviewUsesDefaultThemeAndIsClientCacheable() {
     $this->assertNotEmpty($assert_session->waitForElementVisible('css', 'img[src*="image-test.png"]'));
     $element = $assert_session->elementExists('css', '[data-media-embed-test-active-theme]');
     $this->assertSame('stable', $element->getAttribute('data-media-embed-test-active-theme'));
-
     // Assert that the first preview request transferred >500 B over the wire.
     // Then toggle source mode on and off. This causes the CKEditor widget to be
     // destroyed and then reconstructed. Assert that during this reconstruction,
     // a second request is sent. This second request should have transferred 0
     // bytes: the browser should have cached the response, thus resulting in a
     // much better user experience.
-    $this->assertGreaterThan(500, $this->getLastPreviewRequestTransferSize());
+    $this->assertGreaterThan(500, $this->getInitialPreviewRequestTransferSize());
     $this->pressEditorButton('source');
     $this->assertNotEmpty($assert_session->waitForElement('css', 'textarea.cke_source'));
     $this->pressEditorButton('source');
@@ -1352,6 +1351,26 @@ protected function closeDialog() {
     $this->assertTrue($result);
   }
 
+  /**
+   * Gets the transfer size of the initial preview request.
+   *
+   * @return int
+   *   The size of the bytes transferred.
+   */
+  protected function getInitialPreviewRequestTransferSize() {
+    $this->getSession()->switchToIFrame();
+    $javascript = <<<JS
+(function(){
+  return window.performance
+    .getEntries()
+    .filter(function (entry) {
+      return entry.initiatorType == 'xmlhttprequest' && entry.name.indexOf('/media/test_format/preview') !== -1 && entry.nextHopProtocol == 'http/1.1';
+    })[0].transferSize;
+})()
+JS;
+    return $this->getSession()->evaluateScript($javascript);
+  }
+
   /**
    * Gets the transfer size of the last preview request.
    *