AddForce and Rigidbody 2D - Body doesn't move correct number of units per second?

I have a ratio of 1 Unity unit to 1 pixel set up with my orthographic camera.

Each sprite has a 'Pixels To Units' value of 1 in the Import Settings.

I have attached a Rigidbody 2D component to my sprite and set its mass to 1 and told it to ignore gravity.

I then add a force to the body using AddForce

 o.rigidbody2D.AddForce(Vector2.one * 1);

This is only done once and not each update.

The object should now move at a rate of 1 pixel (unity unit) per second. However it appears to be stationary until it eventually moves 1 pixel around 18 seconds later.

Where have I gone wrong?

EDIT:

Setting the velocity directly works just fine. However, I wish to use AddForce.

 o.rigidbody2D.velocity = Vector2.one;