AmfPHP 2.0 running on PHP 7 & PDO - mysql

Question

AmfPHP 2.0 running on Nginx and PHP 7.0 (FPM), PDO is used as database.
As many users log in to the game, the CPU increases immediately to 100% on all 4 cores due to MySQL. If there are fewer players, the process calms down again and the CPU drops.
Code snippet of PDO connection:
class Database extends PDO {
public static function getConnection() {
$connectionString = sprintf("mysql:dbname=%s;host=%s;charset=utf8", MYSQL_DB, MYSQL_HOST);
try {
$pdo = new PDO($connectionString, MYSQL_USER, MYSQL_PASS);
$pdo->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
return $pdo;
} catch(PDOException $e) {
$error = date("Y-m-d g:i:s a T") . "\Database::getConnection\tError: (" . $e->getCode . ") " . $e->getMessage;
throw new Exception($error);
}
}
// Example
public static function Test($arg) {
try {
$pdo = Database::getConnection();
$stmt = $pdo->prepare("select * from xx where xx = :xx");
$stmt->bindParam(":xx", $arg, PDO::PARAM_STR);
$stmt->execute();
$result = $stmt->fetch(PDO::FETCH_ASSOC);
if ($stmt->rowCount() > 0) {
// execute my code
}
} catch(PDOException $e) {
$error = date("Y-m-d g:i:s a T") . "\Database::Test\tError: (" . $e->getCode . ") " . $e->getMessage;
throw new Exception($error);
}
}
}
For further questions I am at any time available.