Blender Cycle Render slow


i’ve created an animation with blender and after a few tests i want to render it finally. I’m using a GTX 750 TI for CUDA rendering. Usually you can render really fast with this graphiccard.
But somehow it renders really slow at the moment. For a black Full HD image without any lights and objects Blender needs about 17 seconds.

When i load the start-up file (cube with light and camera) and render it with cycles in full hd in only needs about 3 seconds.

Has anyone an idea, how i can speed it up?

Sorry if anything is difficult to understand, my english is not very good.