Character vibrates while moving 30fps

Question

I'm developing a 2.5D mobile game with Unity. In order to move the character back and forward I'm using a piece of code inside the update function:

void Update () {

   if (shouldMove==true){

     transform.Translate(Vector3.forward * 3 * Time.deltaTime);
  }
}

So that code works pretty well when the game is running at 60 fps, but when the fps go down to 30 or less, the character starts to vibrate while moving. I've tried to test the same code with a plane terrain and it worked well, so maybe the problem is the collision between the character's and the terrain's colliders. However, I don't understand why if the fps are high it works well. I've tried both capsule collider and a mesh collider but no one has worked. What do you think? Should I try to use another code?

Edit 1: I'm using a capsule collider and a rigidbody. Should I use a Character Controller?

Answer 1

Sam Bauwens is absolutely right in his response, however, this issue normally is caused due to an excess of objects (specially those which are animated). This can worsen the performance a lot.

You should try to delete some of the objects and try if your character still vibrates. If it doesn't, that means that I'm right. Of course, you won't want to delete objects of your scene, so, you can add an asset such as SmartLOD that deletes the geometry of those objects that are not shown on screen and thus enhances your game's performance.

Hope it helps.