diff --git a/doc/03-cli.md b/doc/03-cli.md
index 7aba323a7..99d4272b8 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -1014,6 +1014,13 @@ similar use case), and need to support proxies, please provide the `CGI_HTTP_PRO
 environment variable instead. See [httpoxy.org](https://httpoxy.org/) for further
 details.
 
+### COMPOSER_MAX_PARALLEL_HTTP
+
+Set to an integer to configure how many files can be downloaded in parallel. This
+defaults to 12 and must be between 1 and 50. If your proxy has issues with
+concurrency maybe you want to lower this. Increasing it should generally not result
+in performance gains.
+
 ### HTTP_PROXY_REQUEST_FULLURI
 
 If you use a proxy, but it does not support the request_fulluri flag, then you
diff --git a/src/Composer/Util/HttpDownloader.php b/src/Composer/Util/HttpDownloader.php
index 519e17c54..757e5f3c6 100644
--- a/src/Composer/Util/HttpDownloader.php
+++ b/src/Composer/Util/HttpDownloader.php
@@ -37,7 +37,7 @@ class HttpDownloader
     private $jobs = array();
     private $options = array();
     private $runningJobs = 0;
-    private $maxJobs = 10;
+    private $maxJobs = 12;
     private $curl;
     private $rfs;
     private $idGen = 0;
@@ -71,6 +71,10 @@ class HttpDownloader
         }
 
         $this->rfs = new RemoteFilesystem($io, $config, $options, $disableTls);
+
+        if (is_numeric($maxJobs = getenv('COMPOSER_MAX_PARALLEL_HTTP'))) {
+            $this->maxJobs = max(1, min(50, (int) $maxJobs));
+        }
     }
 
     /**