I’ve made basic snake game on c++ with qt. I want to adjust the speed of snake by pressing buttons. But when I do this, the snake starts to move uneven, seems like it skipping frames. Initially it works fine with any speed initialised in ‘delay’ variable.

#include "model.h"
#include <QDebug>

s21::Model::Model(){
    initGame();
}

bool s21::Model::getIsInGame(){
    return isInGame;
}

QPoint s21::Model::getApple()
{
    return apple;
}

QVector<QPoint> s21::Model::getDots()
{
    return dots;
}

int s21::Model::getDelay()
{
    return DELAY;
}

void s21::Model::setDelay(int delay)
{
    DELAY = delay;
}

void s21::Model::increaseSpeed()
{
    setDelay(getDelay() - 10);
    restartTimer();
    qDebug() << getDelay();

}

void s21::Model::decreaseSpeed()
{
    setDelay(getDelay() + 10);
    restartTimer();
    qDebug() << getDelay();
}

void s21::Model::timerEvent(QTimerEvent *e)
{
    Q_UNUSED(e);
    if(isInGame){
        checkApple();
        move();
        checkCollision();
    }
}

void s21::Model::locateApple()
{
    apple.rx() = arc4random() % 10;
    apple.ry() = arc4random() % 20;
    emit gameStateChanged();
}

void s21::Model::move()
{
    // action = Right;

    for(int i = dots.size() - 1; i > 0; --i ){
        dots[i] = dots [i-1];
    }

    switch (action) {
    case Left: { dots[0].rx() -= 1; break; }
    case Right: { dots[0].rx() += 1; break; }
    case Up: { dots[0].ry() -= 1; break; }
    case Down: { dots[0].ry() += 1; break; }
    }
    emit gameStateChanged();
}

void s21::Model::checkCollision()
{
    if(dots.size() > 4){
        for(int i = 1; i < dots.size(); ++i){
            if(dots[0] == dots[i]){
                isInGame = false;
            }
        }
    }
    if(dots[0].x() >= 10) { isInGame = false; }
    if(dots[0].x() < 0) { isInGame = false; }
    if(dots[0].y() >= 20) { isInGame = false; }
    if(dots[0].y() <  0) { isInGame = false; }

    if(!isInGame){
        killTimer(timerId );
    }
}

void s21::Model::checkApple()
{
    if (apple == dots[0]){
        dots.push_back(QPoint(0,0));
        locateApple();
    }
}

void s21::Model::restartTimer()
{
    killTimer(timerId);
    timerId = startTimer(DELAY);
}

void s21::Model::initGame(){
    isInGame = true;
    dots.resize(4);
    for(int i = 0; i < dots.size(); ++i){
        dots[i].rx() = dots.size() - i - 1;
        dots[i].ry() = 0;
    }
    timerId = startTimer(DELAY);
    locateApple();
}
`

I’m reading the buttons in view.cpp by this function:
void s21::View::keyPressEvent(QKeyEvent *event)

{
    int key = event->key();
    switch(key) {
    case Qt::Key_Left:
        controller.setDirLeft(key);
        break;
    case Qt::Key_Right:
        controller.setDirRight(key);
        break;
    case Qt::Key_Up:
        controller.setDirUp(key);
        break;
    case Qt::Key_Down:
        controller.setDirDown(key);
        break;
    case Qt::Key_E:
        controller.speedUp(key);
        break;
    case Qt::Key_Q:
        controller.slowDown(key);
        break;
    }
}

I also logged time on every move, originally it works fine but when i change speed by buttons, for example to delay = 240, the time doesn’t correspond to the delay:

QTime("15:51:36.607")
QTime("15:51:36.760")
QTime("15:51:36.832")
QTime("15:51:36.928")
QTime("15:51:37.083")
QTime("15:51:37.083")
QTime("15:51:37.235")
QTime("15:51:37.332")
QTime("15:51:37.403")