Привет всем. Вот столкнулся с проблемой нагрузки на сервер. Написал тест. Нормально работает )). Но вот загвоздка: сервер, на котором сайт, во время тестирования чуть не упал. На сайте было чуть больше 1,5 т. посетителей.
Привожу код запросов:

function getTest()
    {
        $config = Core::selectCol("SELECT name AS array_key, config FROM configs");
        $teacher = $_SESSION['auth']['id'];

        date_default_timezone_set('Europe/Kiev');

        //echo mktime(20, 25, 0, 10, 9, 2014);

        $this->message['time_start'] = Core::formating_date($config['time_start']);
        $this->message['time_finish'] = Core::formating_date($config['time_finish']);

        if ($config['time_start'] > time()) {
            $this->message['time_begin'] = strtotime($this->message['time_start']) - time();
            $this->user = Core::selectRow("SELECT t.login, t.password, r.name as region, t.email, t.phone
                                            FROM teachers as t
                                            LEFT JOIN geo_regions as r ON t.region = r.id
                                            WHERE t.id = '{$teacher}'
                                          ");
        } elseif ($config['time_finish'] < time()) {
            $this->getResult($teacher);
        } else {
            $this->message['time_left'] = strtotime($this->message['time_finish']) - time();
            $count_questions = Core::selectRow("SELECT count(id) AS count_id FROM test_questions");

            if ($_GET['id'] > $count_questions['count_id'] || !isset($_GET['id']) || $_GET['id'] == NULL)
                $_GET['id'] = 1;

            $id = $_GET['id'];

            $answers = Core::select("SELECT id, questions FROM test_answers WHERE teachers = '{$teacher}' ORDER BY questions");

            $missed = array();

            if (!empty($answers)) {
                foreach ($answers as $answers_val) {
                    if ($answers_val['questions'] == $id) {
                        $id++;
                    }
                    $missed[] = $answers_val['questions'];
                }
            }

            $this->missed = Core::select("SELECT id FROM test_questions WHERE id < '{$id}'");

            if (!empty($this->missed)) {
                foreach ($this->missed as $key => $val) {
                    foreach ($missed as $missed_val) {
                        if ($val['id'] == $missed_val) unset($this->missed[$key]);
                    }
                }
            }

            if (count($answers) < $count_questions['count_id']) {
                $this->questions = Core::selectRow("SELECT * FROM test_questions WHERE id = '{$id}'");

                if (in_array($id, $missed) > 0 || $this->questions['id'] < 1) {
                    header("Location:?option=teacher&id=1");
                }
            } else {
                $this->message['time_result'] = "бла-бла " . $this->message['time_finish'];
            }
        }
    }

Сохранение:

function postTeacher()
    {
        $teachers = (int)$_SESSION['auth']['id'];
        $questions = (int)$_GET['id'] - 1;

        if ($_POST['answer'] == NULL) $_POST['answer'] = 'skip';
        $answers = $_POST['answer'];

        unset($_POST);

        $answer = Core::selectRow("SELECT id FROM test_answers WHERE teachers = '{$teachers}' AND questions = '{$questions}'");

        if ($teachers > 0 && $questions > 0 && $answers != NULL && $answer == NULL) {
            Core::query("INSERT INTO test_answers (teachers, questions, answers) VALUES ('$teachers', '$questions', '$answers')");
        } else {
            $this->alert['info'] = "бла-бла!";
        }
    }

Функции для работы с БД:

public function select($sql)
    {
        global $db;
        $result = $db->query($sql);
        $this->query_error($sql);
        if ($result->num_rows > 0) {
            $list = '';
            while ($row = $result->fetch_assoc()) {
                $list[] = $row;
            }
            return $list;
        } else {
            return false;
        }
    }

    public function selectRow($sql)
    {
        global $db;
        $result = $db->query($sql);
        $this->query_error($sql);
        if ($result->num_rows > 0) {
            $row = $result->fetch_assoc();
            return $row;
        } else {
            return false;
        }
    }

    public function selectCol($sql)
    {
        global $db;
        $result = $db->query($sql);
        $this->query_error($sql);
        if ($result->num_rows > 0) {
            $list = '';
            while ($row = $result->fetch_array()) {
                $list[$row['array_key']] = $row['1'];
            }
            return $list;
        } else {
            return false;
        }
    }

    public function query($sql)
    {
        global $db;
        $result = $db->query($sql);
        $this->query_error($sql);
        if ($result) {
            return $result;
        } else {
            return false;
        }
    }

Может кто-то поможет оптимизировать запросы, либо скажет в чем проблема может быть? Сервер германский (недешевый). Если нужна дополнительная информация, пишите.