diff --git a/bench/phpunit_env_bench.php b/bench/phpunit_env_bench.php
new file mode 100644
index 00000000000..e6da923f89d
--- /dev/null
+++ b/bench/phpunit_env_bench.php
@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Benchmark for StaticPHPUnitEnvironment::isPHPUnitRun().
+ *
+ * The method answers a per-process constant (PHPUNIT_COMPOSER_INSTALL is defined once at phpunit
+ * bootstrap, never after), yet it is called hundreds of thousands of times per Rector run -
+ * Skipper::resolveSkippedClasses() alone calls it once per skip resolution. Without memoization
+ * each call pays a defined() lookup.
+ *
+ * Run on both branches to compare:
+ *   php bench/phpunit_env_bench.php
+ *
+ * Reports CPU time (user+sys) and wall time; CPU is the reliable metric, wall is noisy
+ * under concurrent load.
+ */
+
+use Rector\Testing\PHPUnit\StaticPHPUnitEnvironment;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+const CALLS = 255_000;  // calls per run, matching a real run's isPHPUnitRun() volume
+const REPEAT = 200;     // outer repeats
+
+$cpuSeconds = static function (): float {
+    $usage = getrusage();
+    return $usage['ru_utime.tv_sec'] + $usage['ru_utime.tv_usec'] / 1_000_000
+        + $usage['ru_stime.tv_sec'] + $usage['ru_stime.tv_usec'] / 1_000_000;
+};
+
+$run = static function (): void {
+    for ($i = 0; $i < CALLS; ++$i) {
+        StaticPHPUnitEnvironment::isPHPUnitRun();
+    }
+};
+
+// warm up once, excluded from timing
+$run();
+
+$cpuStart = $cpuSeconds();
+$wallStart = hrtime(true);
+
+for ($repeat = 0; $repeat < REPEAT; ++$repeat) {
+    $run();
+}
+
+printf('StaticPHPUnitEnvironment::isPHPUnitRun benchmark%s', PHP_EOL);
+printf('  calls=%d repeat=%d%s', CALLS, REPEAT, PHP_EOL);
+printf('%s', PHP_EOL);
+printf('  isPHPUnitRun  cpu=%.4fs  wall=%.4fs%s', $cpuSeconds() - $cpuStart, (hrtime(true) - $wallStart) / 1_000_000_000, PHP_EOL);
diff --git a/src/Testing/PHPUnit/StaticPHPUnitEnvironment.php b/src/Testing/PHPUnit/StaticPHPUnitEnvironment.php
index 08b13fc1620..29a56a2c608 100644
--- a/src/Testing/PHPUnit/StaticPHPUnitEnvironment.php
+++ b/src/Testing/PHPUnit/StaticPHPUnitEnvironment.php
@@ -6,11 +6,15 @@
 
 final class StaticPHPUnitEnvironment
 {
+    // the constant is defined once at phpunit bootstrap and never changes within a process,
+    // so the lookup is memoized - isPHPUnitRun() is called hundreds of thousands of times per run
+    private static ?bool $isPHPUnitRun = null;
+
     /**
      * Never ever used static methods if possible, this is just handy for tests + src to prevent duplication.
      */
     public static function isPHPUnitRun(): bool
     {
-        return defined('PHPUNIT_COMPOSER_INSTALL');
+        return self::$isPHPUnitRun ??= defined('PHPUNIT_COMPOSER_INSTALL');
     }
 }