diff --git a/php-transformer/tests/run.php b/php-transformer/tests/run.php
index ad1484b22..b86c9efc6 100644
--- a/php-transformer/tests/run.php
+++ b/php-transformer/tests/run.php
@@ -4,7 +4,8 @@
  *
  * Each script is its own process and signals failure with a nonzero exit
  * code. Files ending in `-fixture.php` are builders that tests require, not
- * tests, and are skipped.
+ * tests, and are skipped. Files ending in `-scaling.php` compare their own
+ * timings, so they run one at a time after the parallel batch.
  *
  * Usage: php tests/run.php [--jobs=N] [dir ...]
  * Defaults: tests/contract tests/unit, one job per CPU.
@@ -34,38 +35,53 @@
     }
 }
 sort($tests);
+$scaling = array_values(array_filter($tests, static fn (string $test): bool => str_ends_with($test, '-scaling.php')));
 
-$queue = $tests;
-$running = array();
-$failures = array();
-$durations = array();
-$started = microtime(true);
-
-while ( array() !== $queue || array() !== $running ) {
-    while ( count($running) < $jobs && array() !== $queue ) {
-        $test = array_shift($queue);
-        $log = tmpfile();
-        $process = proc_open(array(PHP_BINARY, $test), array(0 => array('file', '/dev/null', 'r'), 1 => $log, 2 => $log), $pipes, $root);
-        $running[] = array('test' => $test, 'process' => $process, 'log' => $log, 'start' => microtime(true));
-    }
-    foreach ( $running as $index => $job ) {
-        $status = proc_get_status($job['process']);
-        if ( $status['running'] ) {
-            continue;
+/**
+ * Runs the tests with at most $jobs at once.
+ *
+ * @param list<string> $tests
+ * @return array{0: array<string, float>, 1: array<string, string>} Durations and failure output, keyed by test.
+ */
+function run_tests(array $tests, int $jobs, string $root): array
+{
+    $running = array();
+    $failures = array();
+    $durations = array();
+    while ( array() !== $tests || array() !== $running ) {
+        while ( count($running) < $jobs && array() !== $tests ) {
+            $test = array_shift($tests);
+            $log = tmpfile();
+            $process = proc_open(array(PHP_BINARY, $test), array(0 => array('file', '/dev/null', 'r'), 1 => $log, 2 => $log), $pipes, $root);
+            $running[] = array('test' => $test, 'process' => $process, 'log' => $log, 'start' => microtime(true));
         }
-        proc_close($job['process']);
-        $durations[$job['test']] = microtime(true) - $job['start'];
-        if ( 0 !== $status['exitcode'] ) {
-            rewind($job['log']);
-            $failures[$job['test']] = (string) stream_get_contents($job['log']);
-            fwrite(STDERR, "FAIL {$job['test']}\n");
+        foreach ( $running as $index => $job ) {
+            $status = proc_get_status($job['process']);
+            if ( $status['running'] ) {
+                continue;
+            }
+            proc_close($job['process']);
+            $durations[$job['test']] = microtime(true) - $job['start'];
+            if ( 0 !== $status['exitcode'] ) {
+                rewind($job['log']);
+                $failures[$job['test']] = (string) stream_get_contents($job['log']);
+                fwrite(STDERR, "FAIL {$job['test']}\n");
+            }
+            fclose($job['log']);
+            unset($running[$index]);
         }
-        fclose($job['log']);
-        unset($running[$index]);
+        usleep(20000);
     }
-    usleep(20000);
+
+    return array($durations, $failures);
 }
 
+$started = microtime(true);
+[$durations, $failures] = run_tests(array_values(array_diff($tests, $scaling)), $jobs, $root);
+[$scalingDurations, $scalingFailures] = run_tests($scaling, 1, $root);
+$durations += $scalingDurations;
+$failures += $scalingFailures;
+
 arsort($durations);
 echo "Slowest:\n";
 foreach ( array_slice($durations, 0, 5, true) as $test => $seconds ) {