curl kullanarak biraz daha hızlandırabilirsin. çok aşırı fark yaratmasada yararı olacaktır.

function vericek($url){
$ch = curl_init();
curl_setopt($ch, CURLOPT_REFERER, 'http://www.google.com');
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$file_data = curl_exec($ch);
curl_close($ch);
return $file_data;
}
$urlyt = lgN6dTJvCvA;
$url = 'https://www.googleapis.com/youtube/v3/videos?part=contentDetails&id='. $urlyt .'&key=anahtar';
$data = vericek($url);
$characters = json_decode($data);
$videokalite = $characters->items[0]->contentDetails->definition;
echo $videokalite ;