diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -1165,5 +1165,5 @@
   }
   else {
-    KeyValueFactory::get('system.module.schema')->set($module, $weight);
+    KeyValueFactory::get('system.module.weight')->set($module, $weight);
   }
 }
diff -u b/core/modules/field/field.module b/core/modules/field/field.module
--- b/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -339,7 +339,10 @@
  * required if there are any active fields of that type.
  */
 function field_system_info_alter(&$info, $file, $type) {
-  if ($type == 'module' && module_hook($file->name, 'field_info')) {
+  // module_hook() relies on the very function this is called from:
+  // system_rebuild_module_data().
+  $function = $file->name . '_field_info';
+  if ($type == 'module' && function_exists($function)) {
     try {
       $fields = field_read_fields(array('module' => $file->name), array('include_deleted' => TRUE));
     }
diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2684,12 +2684,20 @@
   $modules = array();
   // If this is the first time, start with all modules.
   if (empty($module_store)) {
+    // During a test run no modules can move, no info files can change so
+    // speed up test runs by only finding and parsing once.
+    if (drupal_valid_test_ua() && ($data = state()->get(__FUNCTION__)))  {
+      $module_store = $data['module_store'];
+      $profiles_seen = $data['profiles_seen'];
+    }
+  }
+  if (empty($module_store)) {
     // Find modules and installation profiles. As these are files, they can't
     // change within a request. However, it does depend on $profile.
     $modules = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules', 'name', 0);
   }
-  // If this profile is not yet seen, add it.
-  if (empty($profiles_seen[$profile])) {
+  // If this profile is not yet parsed, add it.
+  if (!isset($profiles_seen[$profile])) {
     $profiles = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.profile$/', 'profiles', 'name', 0);
     // Include the install profile in modules that are loaded.
     $modules[$profile] = $profiles[$profile];
@@ -2748,6 +2756,12 @@
     }
     // Store the processed modules.
     $module_store += $modules;
+    if (drupal_valid_test_ua()) {
+      state()->set(__FUNCTION__, array(
+        'module_store' => $module_store,
+        'profiles_seen' => $profiles_seen,
+      ));
+    }
   }
   $modules = $module_store;
   // Do not return the non-active profile modules. Typically at most one: when
@@ -2776,27 +2790,25 @@
     $modules = _system_rebuild_module_data();
     ksort($modules);
     // Add status, weight and schema.
-    $enabled_modules = module_config()->get();
-    $all_data = array(
-      'weight' => KeyValueFactory::get('system.module.weight')->getAll(),
-      'schema_version' => KeyValueFactory::get('system.module.schema')->getAll(),
-    );
     foreach ($modules as $module => $record) {
-      // Set defaults.
+      // Set defaults and name.
+      $record->name = $module;
       $record->weight = 0;
       $record->status = 0;
       $record->schema_version = SCHEMA_UNINSTALLED;
-      // Copy schema version for installed and nonzero weight for disabled.
-      foreach ($all_data as $key => $data) {
-        if (isset($data[$module])) {
-          $record->$key = $data[$module];
-        }
-      }
-      // Copy enabled weight and status.
-      if (isset($enabled_modules[$module])) {
-        $record->status = 1;
-        $record->weight = $enabled_modules[$module];
-      }
+    }
+    // Installed schema versions.
+    foreach (KeyValueFactory::get('system.module.schema')->getAll() as $module => $schema_version) {
+      $modules[$module]->schema_version = $schema_version;
+    }
+    // Disabled weights.
+    foreach (KeyValueFactory::get('system.module.weight')->getAll() as $module => $weight) {
+      $modules[$module]->weight = $weight;
+    }
+    // Enabled status and weights.
+    foreach (module_config()->get() as $module => $weight) {
+      $modules[$module]->status = 1;
+      $modules[$module]->weight = $weight;
     }
     $modules = _module_build_dependencies($modules);
     // As noted above, system_list_reset() deletes $modules_cache so the order
@@ -2804,18 +2816,23 @@
     system_list_reset();
     $modules_cache['data'] = $modules;
   }
-  if (isset($modules_cache['data']) && !isset($modules_cache['info_alter']) && drupal_get_bootstrap_phase() == DRUPAL_BOOTSTRAP_FULL) {
+  if (!isset($modules_cache['info_alter']) && drupal_get_bootstrap_phase() == DRUPAL_BOOTSTRAP_FULL) {
     // Invoke hook_system_info_alter() to give installed modules a chance to
     // modify the data in the .info files if necessary. It is not possible to
     // use drupal_alter() which relies on system_list() which relies on this
     // function.
     $type = 'module';
-    foreach (module_config()->get() as $module) {
+    // It is not impossible a function resets the modules cache. Keep it.
+    $modules = $modules_cache['data'];
+    foreach (module_config()->get() as $module => $weight) {
       $function = $module .'_system_info_alter';
       if (function_exists($function)) {
-        $function($modules_cache['data'][$module]->info, $modules_cache['data'][$module], $type);
+        foreach ($modules as $record) {
+          $function($record->info, $record, $type);
+        }
       }
     }
+    $modules_cache['data'] = $modules;
     $profile = drupal_get_profile();
     if (isset($modules_cache['data'][$profile])) {
       // The install profile is required, if it's a valid module.