I am rendering 20 sec. animation. It takes about 1 minute to render 1 frame. I left animation for rendering overnight. After 3-4 hours it slowed down. It took 12 hours to render 1 frame. Once I come back to check the render window this morning rendering went to normal speed (about 1 minute per frame). I have really simple scene set up. AO/Reflection channels in Texture applied to MoText.
Thank you for your help.
I know this is quite an old post and don't know if you ever resolved this but I was having the same issue. Check your energy saving settings and make sure that your display isn't set to switch off automatically. After doing some research it seems that this has an effect on rendering, not sure why.