diff --git a/packages/class-files-iterator/composer.json b/packages/class-files-iterator/composer.json
index 11735a8d0..2f0f8a0e8 100644
--- a/packages/class-files-iterator/composer.json
+++ b/packages/class-files-iterator/composer.json
@@ -24,6 +24,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^10.5.38"
+        "phpunit/phpunit": "^10.5.38",
+        "symfony/finder": "^7.4"
     }
 }
diff --git a/packages/class-files-iterator/tests/src/PerformanceTest.php b/packages/class-files-iterator/tests/src/PerformanceTest.php
new file mode 100644
index 000000000..7b1bdfba2
--- /dev/null
+++ b/packages/class-files-iterator/tests/src/PerformanceTest.php
@@ -0,0 +1,72 @@
+<?php
+
+namespace Ock\ClassFilesIterator\Tests;
+
+use Ock\ClassFilesIterator\NamespaceDirectory;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Finder\Finder;
+
+class PerformanceTest extends TestCase {
+
+  public function testPerformance(): void {
+    $nsdir = NamespaceDirectory::fromKnownClass(TestCase::class)->requireParentN(1);
+    $dir = $nsdir->getDirectory();
+    $tick = $this->stopwatch();
+    $dts = [];
+
+    for ($i = 0; $i < 10; ++$i) {
+      $it = new \RecursiveDirectoryIterator($dir, \FilesystemIterator::SKIP_DOTS);
+      $itt = new \RecursiveIteratorIterator($it, \RecursiveIteratorIterator::CHILD_FIRST);
+      $itt_files = iterator_to_array($itt);
+      $dts['itt'][] = $tick();
+
+      $finder = Finder::create()->files()->name('*.php')->in($dir);
+      $n_finder = iterator_count($finder);
+      $dts['finder'][] = $tick();
+
+      $n_nsdir = iterator_count($nsdir);
+      $dts['nsdir'][] = $tick();
+    }
+
+    $itt_php_files = preg_grep('#\.php$#', $itt_files);
+    assert($itt_php_files !== false);
+    $n_itt = count($itt_php_files);
+    $this->assertGreaterThan(300, $n_itt);
+    $this->assertLessThan(3000, $n_itt);
+    $this->assertSame($n_itt, $n_finder);
+    $this->assertSame($n_itt, $n_nsdir);
+
+    $dt_counts = array_map(count(...), $dts);
+    $this->assertSame([], array_diff($dt_counts, [10]));
+
+    // For a deterministic operation, the fastest time is usually the most
+    // reproducible and comparable.
+    // @phpstan-ignore argument.type
+    $dt_mins = array_map(min(...), $dts);
+
+    $dt_ref = $dt_mins['itt'];
+    $this->assertEqualsWithDeltaFactor(5.4, $dt_mins['finder'] / $dt_ref, .1);
+    $this->assertEqualsWithDeltaFactor(3.17, $dt_mins['nsdir'] / $dt_ref, .1);
+  }
+
+  protected function assertEqualsWithDeltaFactor(float $expected, float $actual, float $delta): void {
+    $this->assertLessThan($expected * (1 + $delta), $actual);
+    $this->assertGreaterThan($expected / (1 + $delta), $actual);
+  }
+
+  /**
+   * Starts a stopwatch timer.
+   *
+   * @return \Closure(): float
+   *   Function which returns milliseconds since previous call.
+   */
+  protected function stopwatch(): \Closure {
+    $t0 = hrtime(true);
+    return function () use (&$t0) {
+      $dt = hrtime(true) - $t0;
+      $t0 += $dt;
+      return $dt * .000001;
+    };
+  }
+
+}