field_sync_field_status() needlessly rebuilds module data, slows down installation dramatically.

field_sync_field_status() runs for each module enabled / installed.
It uses system_rebuild_module_data(), which means that the data for all modules needs to be rebuilt (and all of their info files reparsed).
This gets old really fast. On an install profile with 133 modules (Kickstart v2), it adds minutes to the profile install time.

Original issue

The call to system_rebuild_module_data() in field_sync_field_status() is unnecessary and slows down installation noticeably due to the cache rebuilding taking place.

You might see #1599146: field_modules_enabled() greatly slows down profile installs, especially with features modules included and #1574716: Avoid unnecessary cache rebuilds when creating and updating fields for earlier discussion.

Here I am trying out a suggestion by bojanz from those threads, which is to do something like this - new code in the if-block, old code in the else-block for clarity:

/**
 * Refreshes the 'active' and 'storage_active' columns for fields.
 */
function field_sync_field_status() {
  // Refresh the 'active' and 'storage_active' columns according to the current
  // set of enabled modules.
  //
  // But don't run the full system_rebuild_module_data() during installation,
  // as it is slow.
  if (variable_get('install_task') != 'done') {
    $modules = module_list(TRUE);
    foreach ($modules as $module) {
      field_associate_fields($module);
    }
  }
  else {
    $all_modules = system_rebuild_module_data();
    $modules = array();
    foreach ($all_modules as $module_name => $module) {
      if ($module->status) {
        $modules[] = $module_name;
        field_associate_fields($module_name);
      }
    }
  }
  db_update('field_config')
    ->fields(array('active' => 0))
    ->condition('module', $modules, 'NOT IN')
    ->execute();
  db_update('field_config')
    ->fields(array('storage_active' => 0))
    ->condition('storage_module', $modules, 'NOT IN')
    ->execute();
}

For our large install profile, we found that this was a good speed-up and did not cause any unexpected issues.