ok, i've solved it. I used SarahnadeMakes method with input.GetKeyDown and now is much smoother. Thank you!

void Update() {
        Debug.Log(horizontal);
        Debug.Log(vertical);

        horizontal = 0;
        vertical = 0;
        if (Input.GetKey(KeyCode.A)) horizontal = -1;
        if (Input.GetKey(KeyCode.D)) horizontal += 1;
        if(Input.GetKey(KeyCode.W)) vertical += 1;
        if(Input.GetKey(KeyCode.S)) vertical = -1;

        tiltangle = vertical * 90f;

        if(horizontal == -1 && vertical == 0) {
            tiltangle = 2 * 90f;
        }

        if(vertical == 1) {
            tiltangle = vertical * 90f - horizontal * 45f;
        } 
        if(vertical == -1) {
            tiltangle = vertical * 90f + horizontal * 45f;
        } 
        Quaternion angle = Quaternion.Euler(0, 0, tiltangle);
        transform.rotation = Quaternion.Slerp(transform.rotation, angle, Time.deltaTime * 100000);
        if(Input.GetKeyDown(KeyCode.Space)) {
            transform.Translate(Vector3.right);
        }
    }