Form submission handler for system_performance_settings().
function system_performance_settings_submit($form, &$form_state) {
$config = config('system.performance');
$config
->set('cache.page.enabled', $form_state['values']['cache']);
$config
->set('cache.page.max_age', $form_state['values']['page_cache_maximum_age']);
$config
->set('response.gzip', $form_state['values']['page_compression']);
$config
->set('css.preprocess', $form_state['values']['preprocess_css']);
$config
->set('js.preprocess', $form_state['values']['preprocess_js']);
$config
->save();
}