Time duration for your boost and slow, I think it is the best solution

[Dmytro_Podolynnyi]

public class Driver : MonoBehaviour
{
[SerializeField]
private float _steerSpeed = 300f;

[SerializeField]
private float _moveSpeed = _defaultMoveSpeed;

private const float _defaultMoveSpeed = 20f;

[SerializeField]
private float _slowSpeed = 15f;

[SerializeField]
private float _boostSpeed = 30f;

private const int _duration = 3;

void Update()
{
    float steerAmount = Input.GetAxis("Horizontal") * _steerSpeed * Time.deltaTime;
    float moveAmount = Input.GetAxis("Vertical") * _moveSpeed * Time.deltaTime;
    transform.Rotate(0, 0, -steerAmount);
    transform.Translate(0, moveAmount, 0);
}

private void OnTriggerEnter2D(Collider2D collision)
{
    if(collision.tag == "SpeedUp")
    {
        Debug.Log("Ka-Chow!");
        _moveSpeed = _boostSpeed;

        var thread = new Thread(new ThreadStart(dur));
        thread.Start();
    }
}

private void dur()
{
    Thread.Sleep(_duration * 1000);

    _moveSpeed = _defaultMoveSpeed;
}

private void OnCollisionEnter2D(Collision2D collision)
{
    _moveSpeed = _slowSpeed;

    var thread = new Thread(new ThreadStart(dur));
    thread.Start();
}

}

[Nina]

Good job, Dmytro, and thanks for sharing your solution.