Best Solution for Constant Horizontal Speed

I have a 2D object that will Translate to the left at a constant speed.

void Update()
{
    obstacleObject.transform.Translate(Vector3.left*(1*Time.DeltaTime),Space.World);
}

The problem is that it increases in speed as the game runs. I know I’ve made this work in the past but I’m having a trouble remembering the process.

Thanks!

float speed = 5f;
// Update is called once per frame
void Update()
{
obstacleObject.transform.Translate(new Vector3(-(speed * Time.deltaTime),0f,0f));
}