I am not sure , It might have to do with the angle, you need a higher calculation for a floor ( Horizontal laying objects ) It is just a texture, at the same time it is influenced with your light setup. Try once in your home to change your angle on a table or a horizontal plane, it is consistent, If you focus what is close it will be sharp and get blurry into the far. If a glas stands there you will focus the glass but the edge close to you will be blurry.

I guess that the render needs more to calculate the Horizontal angle as the texture will distort in Plain view  it would have to render live just like in a game or Iray in DS but this will slow down your motion big time . Rendering a Preview will just be some sort of smoothed out screenshot not more . If you want it better and fast then just lower the FF render engine to a minimum ( Draft ) and render the Image