Possible to have constant speed?

I’m making a platformer and it is vital that the speed remains constant…Currently I use

void Update()
transform.position = new Vector2 (transform.position.x + (Speed * Time.deltaTime), transform.position.y);

For the past week or so it was working fine, but recently when I test the level it will lag for the first 3 seconds or so (Im unsure why) and then whether it continues to be going at a constant speed appears to be random. Ive tried alot of methods and the more I research the more I realize that this method should be perfect…or at least not more than a few milliseconds different each playthrough.

So is there even a way in which speed will be constant regardless of lag? Or is this method correct, and It just appears to be lagging in some weird way for me?(If it is a problem with my computer, any clue on why it is only lagging recently and not the past couple of weeks?)

Update does not assure constant execution frequency, maybe you add more assets to your scene and initial lag is due to its loading etc. Move your code to FixedUpdate() and see how it performs.