Lag after parenting objects

Hey, so i'm modelling the chair from blenderguru. And once i parent my objects together whenever i move the object it starts lagging is there anyway i can stop this ? I have a good graphics card and cpu so i dont know what the problem is. I am kinda new to blender so.

So after further investigation i noticed it starts lagging when i add a normal map.
How can this be ?

Ok found it mirror was not aplyd.