diff --git a/core/lib/Drupal/Core/Flood/MemoryBackend.php b/core/lib/Drupal/Core/Flood/MemoryBackend.php
index 9899e35e7ee..062471b3db8 100644
--- a/core/lib/Drupal/Core/Flood/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Flood/MemoryBackend.php
@@ -41,7 +41,7 @@ class MemoryBackend implements FloodInterface {
     // We can't use REQUEST_TIME here, because that would not guarantee
     // uniqueness.
     $time = microtime(TRUE);
-    $this->events[$name][$identifier][$time + $window] = $time;
+    $this->events[$name][$identifier][] = ['expire' => $time + $window, 'time' => $time];
   }
 
   /**
@@ -65,8 +65,8 @@ class MemoryBackend implements FloodInterface {
       return $threshold > 0;
     }
     $limit = microtime(TRUE) - $window;
-    $number = count(array_filter($this->events[$name][$identifier], function ($timestamp) use ($limit) {
-      return $timestamp > $limit;
+    $number = count(array_filter($this->events[$name][$identifier], function ($entry) use ($limit) {
+      return $entry['time'] > $limit;
     }));
     return ($number < $threshold);
   }
@@ -76,12 +76,10 @@ class MemoryBackend implements FloodInterface {
    */
   public function garbageCollection() {
     foreach ($this->events as $name => $identifiers) {
-      foreach ($this->events[$name] as $identifier => $timestamps) {
-        // Filter by key (expiration) but preserve key => value  associations.
-        $this->events[$name][$identifier] = array_filter($timestamps, function () use (&$timestamps) {
-          $expiration = key($timestamps);
-          next($timestamps);
-          return $expiration > microtime(TRUE);
+      foreach ($this->events[$name] as $identifier => $entries) {
+        // Remove expired entries.
+        $this->events[$name][$identifier] = array_filter($entries, function ($entry) {
+          return $entry['expire'] > microtime(TRUE);
         });
       }
     }
diff --git a/core/modules/system/tests/src/Kernel/System/FloodTest.php b/core/modules/system/tests/src/Kernel/System/FloodTest.php
index cf32fd9fed7..97b78aa974e 100644
--- a/core/modules/system/tests/src/Kernel/System/FloodTest.php
+++ b/core/modules/system/tests/src/Kernel/System/FloodTest.php
@@ -74,6 +74,18 @@ class FloodTest extends KernelTestBase {
     $this->assertFalse($flood->isAllowed($name, $threshold));
   }
 
+  /**
+   * Tests memory backend records events to the nearest microsecond.
+   */
+  public function testMemoryBackendThreshold() {
+    $request_stack = \Drupal::service('request_stack');
+    $flood = new MemoryBackend($request_stack);
+    $flood->register('new event');
+    $this->assertTrue($flood->isAllowed('new event', '2'));
+    $flood->register('new event');
+    $this->assertFalse($flood->isAllowed('new event', '2'));
+  }
+
   /**
    * Tests flood control database backend.
    */