How to shorten TTFB time?

Question

Yii

Hello. There is a city search form, when entering, an ajax request is sent and a drop-down list appears (5 cities). Form rendered in controller

return $this->renderPartial($viewFile, [
    'cities' => $foundCities
]);

and then output in case of 200th response

resultBox.find('.scrollbar-search-form-result').html(data);

What can be done to reduce TTFB?