diff --git a/src/Composer/Package/Version/VersionGuesser.php b/src/Composer/Package/Version/VersionGuesser.php
index 7e7a0cae1..f3b857375 100644
--- a/src/Composer/Package/Version/VersionGuesser.php
+++ b/src/Composer/Package/Version/VersionGuesser.php
@@ -21,6 +21,8 @@ use Composer\Util\Git as GitUtil;
 use Composer\Util\HttpDownloader;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Svn as SvnUtil;
+use React\Promise\CancellablePromiseInterface;
+use Symfony\Component\Process\Process;
 
 /**
  * Try to guess the current version number based on different VCS configuration.
@@ -307,6 +309,8 @@ class VersionGuesser
                 return strnatcasecmp($b, $a);
             });
 
+            $promises = [];
+            $this->process->setMaxJobs(30);
             foreach ($branches as $candidate) {
                 $candidateVersion = Preg::replace('{^remotes/\S+/}', '', $candidate);
 
@@ -316,19 +320,29 @@ class VersionGuesser
                 }
 
                 $cmdLine = str_replace(array('%candidate%', '%branch%'), array($candidate, $branch), $scmCmdline);
-                if (0 !== $this->process->execute($cmdLine, $output, $path)) {
-                    continue;
-                }
+                $promises[] = $this->process->executeAsync($cmdLine, $path)->then(function (Process $process) use (&$length, &$version, &$prettyVersion, $candidateVersion, &$promises): void {
+                    if (!$process->isSuccessful()) {
+                        return;
+                    }
 
-                if (strlen($output) < $length) {
-                    $length = strlen($output);
-                    $version = $this->versionParser->normalizeBranch($candidateVersion);
-                    $prettyVersion = 'dev-' . $candidateVersion;
-                    if ($length === 0) {
-                        break;
+                    $output = $process->getOutput();
+                    if (strlen($output) < $length) {
+                        $length = strlen($output);
+                        $version = $this->versionParser->normalizeBranch($candidateVersion);
+                        $prettyVersion = 'dev-' . $candidateVersion;
+                        if ($length === 0) {
+                            foreach ($promises as $promise) {
+                                if ($promise instanceof CancellablePromiseInterface) {
+                                    $promise->cancel();
+                                }
+                            }
+                        }
                     }
-                }
+                });
             }
+
+            $this->process->wait();
+            $this->process->resetMaxJobs();
         }
 
         return array('version' => $version, 'pretty_version' => $prettyVersion);
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index d2572a02e..a4c707cf9 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -271,6 +271,16 @@ class ProcessExecutor
         }
     }
 
+    public function setMaxJobs(int $maxJobs): void
+    {
+        $this->maxJobs = $maxJobs;
+    }
+
+    public function resetMaxJobs(): void
+    {
+        $this->maxJobs = 10;
+    }
+
     /**
      * @param  ?int $index job id
      * @return void
@@ -278,7 +288,7 @@ class ProcessExecutor
     public function wait($index = null): void
     {
         while (true) {
-            if (!$this->countActiveJobs($index)) {
+            if (0 === $this->countActiveJobs($index)) {
                 return;
             }
 
diff --git a/tests/Composer/Test/Mock/ProcessExecutorMock.php b/tests/Composer/Test/Mock/ProcessExecutorMock.php
index b8948e6ae..253f1a129 100644
--- a/tests/Composer/Test/Mock/ProcessExecutorMock.php
+++ b/tests/Composer/Test/Mock/ProcessExecutorMock.php
@@ -12,6 +12,8 @@
 
 namespace Composer\Test\Mock;
 
+use Composer\Test\TestCase;
+use PHPUnit\Framework\MockObject\MockBuilder;
 use React\Promise\PromiseInterface;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Platform;
@@ -41,6 +43,19 @@ class ProcessExecutorMock extends ProcessExecutor
      * @var string[]
      */
     private $log = array();
+    /**
+     * @var MockBuilder<Process>
+     */
+    private $processMockBuilder;
+
+    /**
+     * @param MockBuilder<Process> $processMockBuilder
+     */
+    public function __construct(MockBuilder $processMockBuilder)
+    {
+        parent::__construct();
+        $this->processMockBuilder = $processMockBuilder->disableOriginalConstructor();
+    }
 
     /**
      * @param array<string|array{cmd: string|list<string>, return?: int, stdout?: string, stderr?: string, callback?: callable}> $expectations
@@ -180,9 +195,16 @@ class ProcessExecutorMock extends ProcessExecutor
 
     public function executeAsync($command, ?string $cwd = null): PromiseInterface
     {
-        $resolver = function ($resolve, $reject): void {
-            // TODO strictly speaking this should resolve with a mock Process instance here
-            $resolve();
+        $cwd = $cwd ?? Platform::getCwd();
+
+        $resolver = function ($resolve, $reject) use ($command, $cwd): void {
+            $result = $this->doExecute($command, $cwd, false, $output);
+            $procMock = $this->processMockBuilder->getMock();
+            $procMock->method('getOutput')->willReturn($output);
+            $procMock->method('isSuccessful')->willReturn($result === 0);
+            $procMock->method('getExitCode')->willReturn($result);
+
+            $resolve($procMock);
         };
 
         $canceler = function (): void {
diff --git a/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php b/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
index 7f8872fc1..6d0cc3d34 100644
--- a/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
@@ -20,6 +20,7 @@ use Composer\Package\RootPackage;
 use Composer\Package\Version\VersionGuesser;
 use Composer\Semver\VersionParser;
 use Composer\Test\TestCase;
+use Composer\Util\ProcessExecutor;
 
 class RootPackageLoaderTest extends TestCase
 {
@@ -36,8 +37,11 @@ class RootPackageLoaderTest extends TestCase
 
         $config = new Config;
         $config->merge(array('repositories' => array('packagist' => false)));
+        $processExecutor = new ProcessExecutor();
+        $processExecutor->enableAsync();
+        $guesser = new VersionGuesser($config, $processExecutor, new VersionParser());
 
-        $loader = new RootPackageLoader($manager, $config);
+        $loader = new RootPackageLoader($manager, $config, null, $guesser);
 
         return $loader->load($data);
     }
diff --git a/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php b/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
index bcacb53a6..c4e607566 100644
--- a/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
@@ -320,7 +320,7 @@ class GitHubDriverTest extends TestCase
             ->setConstructorArgs(array($io, $this->config))
             ->getMock();
 
-        $gitHubDriver = new GitHubDriver($repoConfig, $io, $this->config, $httpDownloader, new ProcessExecutorMock);
+        $gitHubDriver = new GitHubDriver($repoConfig, $io, $this->config, $httpDownloader, $this->getProcessExecutorMock());
         $gitHubDriver->initialize();
     }
 
diff --git a/tests/Composer/Test/TestCase.php b/tests/Composer/Test/TestCase.php
index a26dbfb6e..ff04e79d1 100644
--- a/tests/Composer/Test/TestCase.php
+++ b/tests/Composer/Test/TestCase.php
@@ -32,6 +32,7 @@ use Composer\Package\RootAliasPackage;
 use Composer\Package\CompletePackage;
 use Composer\Package\CompleteAliasPackage;
 use Composer\Package\Package;
+use Symfony\Component\Process\Process;
 
 abstract class TestCase extends \PHPUnit\Framework\TestCase
 {
@@ -256,7 +257,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
 
     protected function getProcessExecutorMock(): ProcessExecutorMock
     {
-        $this->processExecutorMocks[] = $mock = new ProcessExecutorMock();
+        $this->processExecutorMocks[] = $mock = new ProcessExecutorMock($this->getMockBuilder(Process::class));
 
         return $mock;
     }