Excuse me! In render panel,under sample set,there are final and preview,which it is what? I never choose it when render ever.It must be chosen when render? Thanks.

Preview Render is usually the result when you do a quick or test preview it is not your final render so the quality is usually set to like 50% of what you want your final image result final is where you set how many samples and the size of you want for your final render

Thanks.But it should be slowly render.

Render times depend on your system if you have a GPU enabled graphic card render times are usually faster than CPU it all depends on your systems specs